Again, the two newcomers have decided to mess things up for themselves, in such a way that they can't respond to this fairly simple query (which should return 0 rows in 0.00secs, as on DB-4)
All three servers have same amount data in each table, just checked with select (*) from ... , but look here:

	DB-4 explain: 
	MariaDB [maths]> explain extended SELECT COUNT(1) FROM (SELECT `t1`.`id`, `t1`.`plugin_id`, `t1`.`configuration`, `t1`.`hash`, `t1`.`run_until`, `t1`.`num_runs`, `t1`.`target_type`, `t1`.`interval`, `t1`.`created`, `t1`.`next_run`, `t1`.`latest_run`, `t1`.`status`, `t1`.`owner_id`, `t1`.`created_for_id`, `t1`.`sel`, `t1`.`filters`, `t1`.`tags` FROM `databasename`.`tasks` AS t1 INNER JOIN `databasename`.`users` AS t2 ON (`t1`.`owner_id` = `t2`.`id`) INNER JOIN `databasename`.`users` AS t3 ON (`t1`.`created_for_id` = `t3`.`id`) INNER JOIN `databasename`.`plugins` AS t4 ON (`t1`.`plugin_id` = `t4`.`id`) INNER JOIN `databasename`.`projecttasks` AS t5 ON (`t1`.`id` = `t5`.`task_id`) LEFT OUTER JOIN `maths`.`operation` AS t6 ON (`t4`.`pluginname` = `t6`.`opnet_name`) WHERE ((`t1`.`status` != 'pending') AND (`t1`.`sel` LIKE '1026894432')) GROUP BY `t1`.`id`) AS wrapped_select;
	+------+-------------+------------+--------+-----------------------------------------------------+----------------------+---------+--------------------------------+------+----------+--------------------------------------------------------------+
	| id   | select_type | table      | type   | possible_keys                                       | key                  | key_len | ref                            | rows | filtered | Extra                                                        |
	+------+-------------+------------+--------+-----------------------------------------------------+----------------------+---------+--------------------------------+------+----------+--------------------------------------------------------------+
	|    1 | PRIMARY     | <derived2> | ALL    | NULL                                                | NULL                 | NULL    | NULL                           |   63 |   100.00 |                                                              |
	|    2 | DERIVED     | t1         | range  | PRIMARY,tasks_plugin_id,tasks_owner_id,sel_idx      | sel_idx              | 768     | NULL                           |    1 |   100.00 | Using where; Using temporary; Using filesort                 |
	|    2 | DERIVED     | t4         | eq_ref | PRIMARY                                             | PRIMARY              | 4       | databasename.t1.plugin_id      |    1 |   100.00 |                                                              |
	|    2 | DERIVED     | t2         | eq_ref | PRIMARY                                             | PRIMARY              | 4       | databasename.t1.owner_id       |    1 |   100.00 | Using index                                                  |
	|    2 | DERIVED     | t3         | eq_ref | PRIMARY                                             | PRIMARY              | 4       | databasename.t1.created_for_id |    1 |   100.00 | Using index                                                  |
	|    2 | DERIVED     | t5         | ref    | projecttasks_task_id                                | projecttasks_task_id | 4       | databasename.t1.id             |    1 |   100.00 | Using index                                                  |
	|    2 | DERIVED     | t6         | index  | NULL                                                | opname               | 259     | NULL                           |   63 |   100.00 | Using where; Using index; Using join buffer (flat, BNL join) |
	+------+-------------+------------+--------+-----------------------------------------------------+----------------------+---------+--------------------------------+------+----------+--------------------------------------------------------------+

	DB-5 explain - Now these two servers (again) never return an answer to the same query !? :
	+------+-------------+------------+--------+-----------------------------------------------------+----------------------+---------+-------------------------+------+----------+--------------------------------------------------------------+
	| id   | select_type | table      | type   | possible_keys                                       | key                  | key_len | ref                     | rows | filtered | Extra                                                        |
	+------+-------------+------------+--------+-----------------------------------------------------+----------------------+---------+-------------------------+------+----------+--------------------------------------------------------------+
	|    1 | PRIMARY     | <derived2> | ALL    | NULL                                                | NULL                 | NULL    | NULL                    |   63 |   100.00 |                                                              |
	|    2 | DERIVED     | t2         | index  | PRIMARY                                             | PRIMARY              | 4       | NULL                    |    1 |   100.00 | Using index; Using temporary; Using filesort                 |
	|    2 | DERIVED     | t3         | index  | PRIMARY                                             | PRIMARY              | 4       | NULL                    |    1 |   100.00 | Using index; Using join buffer (flat, BNL join)              |
	|    2 | DERIVED     | t4         | index  | PRIMARY                                             | plugins_pluginname   | 602     | NULL                    |    1 |   100.00 | Using index; Using join buffer (incremental, BNL join)       |
	|    2 | DERIVED     | t5         | index  | projecttasks_task_id                                | projecttasks_task_id | 4       | NULL                    |    1 |   100.00 | Using index; Using join buffer (incremental, BNL join)       |
	|    2 | DERIVED     | t1         | eq_ref | PRIMARY,tasks_plugin_id,tasks_owner_id,sel_idx      | PRIMARY              | 4       | databasename.t5.task_id |    1 |   100.00 | Using where                                                  |
	|    2 | DERIVED     | t6         | index  | NULL                                                | opname               | 259     | NULL                    |   63 |   100.00 | Using where; Using index; Using join buffer (flat, BNL join) |
	+------+-------------+------------+--------+-----------------------------------------------------+----------------------+---------+-------------------------+------+----------+--------------------------------------------------------------+

	DB-6 explain - Now these two servers (again) never return an answer to the same query !? :
	+------+-------------+------------+--------+-----------------------------------------------------+----------------------+---------+-------------------------+------+----------+--------------------------------------------------------------+
	| id   | select_type | table      | type   | possible_keys                                       | key                  | key_len | ref                     | rows | filtered | Extra                                                        |
	+------+-------------+------------+--------+-----------------------------------------------------+----------------------+---------+-------------------------+------+----------+--------------------------------------------------------------+
	|    1 | PRIMARY     | <derived2> | ALL    | NULL                                                | NULL                 | NULL    | NULL                    |   63 |   100.00 |                                                              |
	|    2 | DERIVED     | t2         | index  | PRIMARY                                             | PRIMARY              | 4       | NULL                    |    1 |   100.00 | Using index; Using temporary; Using filesort                 |
	|    2 | DERIVED     | t3         | index  | PRIMARY                                             | PRIMARY              | 4       | NULL                    |    1 |   100.00 | Using index; Using join buffer (flat, BNL join)              |
	|    2 | DERIVED     | t4         | index  | PRIMARY                                             | plugins_pluginname   | 602     | NULL                    |    1 |   100.00 | Using index; Using join buffer (incremental, BNL join)       |
	|    2 | DERIVED     | t5         | index  | projecttasks_task_id                                | projecttasks_task_id | 4       | NULL                    |    1 |   100.00 | Using index; Using join buffer (incremental, BNL join)       |
	|    2 | DERIVED     | t1         | eq_ref | PRIMARY,tasks_plugin_id,tasks_owner_id,sel_idx      | PRIMARY              | 4       | databasename.t5.task_id |    1 |   100.00 | Using where                                                  |
	|    2 | DERIVED     | t6         | index  | NULL                                                | opname               | 259     | NULL                    |   63 |   100.00 | Using where; Using index; Using join buffer (flat, BNL join) |
	+------+-------------+------------+--------+-----------------------------------------------------+----------------------+---------+-------------------------+------+----------+--------------------------------------------------------------+