Have you checked borgbackup docs, FAQ, and open Github issues?

Yes

Is this a BUG / ISSUE report or a QUESTION?

BUG / ISSUE report

System information. For client/server mode post info for both machines.

Not so easy, since borg serve has no support for this?

Your borg version (borg -V).

$ borg --version
borg 1.1.17

Operating system (distribution) and version.

$ uname -a
Linux desktop 5.14.0-4-amd64 #1 SMP Debian 5.14.16-1 (2021-11-03) x86_64 GNU/Linux

Hardware / network configuration, and filesystems used.

How much data is handled by borg?

Full borg commandline that lead to the problem (leave away excludes and passwords)

borg check

Describe the problem you're observing.

Execute borg check and press Ctrl + C. Next operation on the repo yields

Failed to create/acquire the lock /lock.exclusive (timeout).

Can you reproduce the problem? If so, describe how. If not, describe troubleshooting steps you took before opening the issue.

See above.

Include any warning/errors/backtraces from the system logs