Yes - the Web UI looks as if it was completely separated from the command line and nothing works here.
SCRUB from the command also did not work - probably because pool_a was mounted readonly and I could not do anything either.
For now pool_a is as exported and all urgent files are copied to another pool
I am trying different ways to import and repair pool_a but without success, a moment ago I tried to import it by running truenas in the previous version 22.12.4.2 and I get the message:
root@truenas[~]# sudo zpool import -fFX -o altroot=/mnt pool_a
This pool uses the following feature(s) not supported by this system:
com.klarasystems:vdev_zaps_v2
cannot import 'pool_a': unsupported version or feature
so at least we know it is in the new version 
If I don’t think of anything, tomorrow I will destroy the pool and start importing the backup data.
Problem solved thanks @Protopia - after a full backup I deleted the pool and created it again. Thanks to everyone for your help!