SphinxSE lets MariaDB Server restarting

No idea if this is really a bug, but we do not know any further here.

We are running MariaDB server (following S1) with databases using a table of type SphinxSE engine. The Sphinx daemon is running on another server (following S2). On S2 also runs MariaDB as slave, which should keep the data of the S1 updated by replication. The daemon gets the data for different indices from the S2. We switched from MySQL to MariaDB because SphinxSe engine is already integrated.
Our CMS makes queries to SphinxSE-Table in S1, where different indexes can be addressed with an ALTER TABLE and joins the search results with JOIN with other data. This combination also works very well.

Now the problem:
When the table exist the engine type SphinxSE at S1 regularly restarts. On average every 15-20 hours, but without an error message. This will not happen if the SphinxSE table is removed.
Also, the replication fails at irregular (larger) intervals with different error message. For example, "duplicate key" errors. This could be related to restarting the S1, but we have not been able to evaluate that yet. There may also be another issue with replication independent of the sphinx.

We have no idea what the problem described could be, even google searches are not very helpful.

SphinxSE-Table:
CREATE TABLE `KEP_SPHINX` (
`id` BIGINT UNSIGNED NOT NULL,
`weight` int(11) NOT NULL,
`query` varchar(3072) NOT NULL,
KEY `query` (`query`)
) ENGINE=SPHINX DEFAULT CHARSET=utf8mb4
CONNECTION='sphinx://xxx.xxx.xxx.xxx:9312/*'

Sample query:
ALTER TABLE KEP_SPHINX CONNECTION="sphinx://xxx.xxx.xxx.xxx:9312/idx_citylist";
SELECT c.*
FROM KEP_SPHINX s
LEFT JOIN KEP_CITYLIST c ON s.ID = c.ID
WHERE query="Seattle;filter=COUNTRY_ID,13;mode=all;maxmatches=1000;limit=1000;"

Is there any more data needed?

Please assist. Thanks