Ok I have now set up the whole system on a brand new Debian server and migrated the data. But I still have the problem somehow.
I really don’t know what it is
I runinng into issues with cron too and having a hard time to figure it out.
I added my comment to another thread and thought linking would be helpful
I am really struggling, so I link this thread here too.
Same problem here. 
cron.php takes up to 90% of 4GB RAM und 30% of CPU for aprox. 30 secs.
Following tflidd’s advice:
select * from oc_jobs ORDER BY execution_duration DeSC;
+------+-----------------------------------------------------------------+--------------------------------------------------------------------------+------------+--------------+-------------+--------------------+
| id | class | argument | last_run | last_checked | reserved_at | execution_duration |
+------+-----------------------------------------------------------------+--------------------------------------------------------------------------+------------+--------------+-------------+--------------------+
| 24 | OCA\UpdateNotification\Notification\BackgroundJob | null | 1617940802 | 1618023302 | 0 | 27 |
| 1 | OCA\NextcloudAnnouncements\Cron\Crawler | null | 1618001701 | 1618023302 | 0 | 1 |
| 44 | OCA\Passwords\Cron\CheckPasswordsJob | null | 1617940829 | 1618023302 | 0 | 1 |
| 48 | OCA\Passwords\Cron\BackupJob | null | 1617940830 | 1618023302 | 0 | 1 |
| 68 | OCA\Deck\Cron\ScheduledNotifications | null | 1618023302 | 1618023302 | 0 | 0 |
| 30 | OCA\Talk\BackgroundJob\ExpireSignalingMessage | null | 1618023002 | 1618023302 | 0 | 0 |
| 31 | OCA\Talk\BackgroundJob\RemoveEmptyRooms | null | 1618023002 | 1618023302 | 0 | 0 |
Except for OCA\UpdateNotification\Notification\BackgroundJob everything fine.
sudo -u www-data php /var/www/vhosts/XXX/public/occ background:queue:status -vvv:
takes long, then “Getötet” - probably “Killed” in English 
Same happens with:
sudo -u www-data php /var/www/vhosts/XXX/public/occ background:queue:status -vvv
Amy ideas? Thanks so much indeed.
Hm… Sounds like [NC 21] OCC Update error: allowed memory size exhausted · Issue #25742 · nextcloud/server · GitHub
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szaimen, you were right! Following this advice, by adding:
'debug' => 'false', // note: string not bool
'loglevel' => 2,
to config/config.php solved the problem.
Thanks!
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I found a problem with the Maps app in my NC installation. Had over 3650 lines of Maps AddPhotoJob lines in my oc_jobs (the cron job table for NC) table.
I uninstalled the Maps app and deleted the jobs from my oc_jobs table with:
DELETE FROM oc_jobs WHERE class LIKE ‘%Maps%’ and argument LIKE ‘%photoId%’;
Now my cron runs through in seconds and uses less than 200 MB of memory.
That helped a lot, especially on a Raspberry Pi, which started swapping a few Gigabytes of RAM everytime the cronjob ran through. 
You can also try this by hand then:
sudo -u www-data php -d memory_limit=512M -f /var/www/nextcloud/cron.php --define apc.enable_cli=1
With limited the php memory usage down to 512 MB. When the Maps AddPhotoJobs were still in, this command crashed with:
“PHP Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 49752824 bytes) in /var/www/nextcloud/apps/maps/lib/Helper”
Which was also a good indication where the whole RAM usage came from
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