Windows service fails with old data

I've migrated my data from MySQL 5.5 to MariaDB 10.0.1|2 (one after the other) by replacing the data/ directory with the mysql data/ directory, and putting MariaDB's my.ini in there. Launching the service failed with error 1067 on first try (with 10.0.1), and error 1053 (I think) on the second try (with 10.0.2).

I could launch mysqld from the command line without any issues (without admin rights), though, and could then run the mysql_upgrade commmand which successfully converted the databases.

Afterwards, launching the service worked just fine.