Downgrading mariadb (including -clients and -libs) to 10.4.14 did bring akonadi back to a working state, so it definitely is the new version.
Running the upgrade-command mentioned in the pacman output didn’t work after re-upgrading.
Apparently i’m not the only one: https://www.reddit.com/r/kde/comments/icz14s/akonadi_wont_start_after_mariadb_update/
I’ve also noticed that running mysqld itself with the given arguments works without errors:
mysqld --defaults-file=/home/termy/.local/share/akonadi/mysql.conf --datadir=/home/termy/.local/share/akonadi/db_data/ --socket=/run/user/1000/akonadi/mysql.socket --pid-file=/run/user/1000/akonadi/mysql.pid [130]
2020-08-22 16:16:36 0 [Note] mysqld (mysqld 10.5.5-MariaDB) starting as process 8932 …
after that, akonadi starts up just fine.
i ran mariadb-upgrade /run/user/1000/akonadi/mysql.socket as mentioned in the reddit-post which did complete fine.
When killing mysql, stopping akonadi and then trying to start akonadi again, i’m back to the error org.kde.pim.akonadiserver: process error: “execvp: Permission denied”