Resolved [Solved] Plesk CP returns error 500

[mit]

Hey there.

My setup:
Ubuntu 18.04.6 LTS
Plesk Obsidian Version 18.0.38 Update #3

Today, I encountered the following issue (maybe this helps someone else):

Trying to access Plesk CP returned error 500 (no page shown, only got the error code).
My websites / mail services etc. were still available (fully, I believe).

Error logs

systemctl status mysql.service
DB query failed: SQLSTATE[HY000] [2002] No such file or directory

/var/log/mysql/error.log
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: innodb_empty_free_list_algorithm has been changed to legacy because of small buffer pool size. In order to use backoff, increase buffer pool at least up to 20MB.
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: Using mutexes to ref count buffer pool pages
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: The InnoDB memory heap is disabled
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: GCC builtin __atomic_thread_fence() is used for memory barrier
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: Compressed tables use zlib 1.2.11
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: Using Linux native AIO
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: Using SSE crc32 instructions
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: Initializing buffer pool, size = 128.0M
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: Completed initialization of buffer pool
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: Highest supported file format is Barracuda.
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: 128 rollback segment(s) are active.
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: Waiting for purge to start
2021-10-10 0:51:36 140176415194240 [Note] InnoDB: Percona XtraDB (Experts in Database Performance Management) 5.6.49-89.0 started; log sequence number 1951518509
2021-10-10 0:51:36 140176415194240 [Note] Plugin 'FEEDBACK' is disabled.
2021-10-10 0:51:36 140176415194240 [Note] Recovering after a crash using tc.log
2021-10-10 0:51:36 140176415194240 [ERROR] Can't init tc log
2021-10-10 0:51:36 140176415194240 [ERROR] Aborting

I managed to get Plesk working by deleting the TC log.
rm -f /var/lib/mysql/tc.log

No recent change done by me on Plesk nor OS level (besides installing updates weekly and having Plesk auto update enabled).

Cheers,
M.