Now I would like to know what might not work because of deleting 'type=3' content or in other words: what is the table and the content for?

The config table stores various system parameters. Deleting them will result in fallback to default values / behavior as specified in config_inc.php / config_default_inc.php.
Not knowing exactly what you deleted, it is not possible to answer your question. It could mess up with your workflow settings, colors, custom status and many other things.

I would not recommend this "carpet-bombing" approach of blindly deleting all type 3 records, you would be much better off analyzing the root cause of each error, and fix them indivdually. You might want to try the upgrade again using a nightly build (http://mantisbt.org/builds.php), as the installer has been improved to provide more helpful error messages, helping you identify the offending data.

Thank you, that's why I'm testing the migration and upgrade process in a VM with Snapshots first.
I tried the upgrade again. This time to v2.16.0 (mantisbt-2.16.0-dev-master-98ea423) to get more debug information.

The error was in "status_enum_workflow". I had to replace the german umlaut (ä, ö, ü) by ae, oe and ue.
Now I get an error in "default_notify_flags" (project_id='0')
config_id project_id user_id access_reqd type value
default_notify_flags 0 0 90 3 {"reporter":1,"handler":1,"threshold_min":100,"threshold_max":0}
Do you have a proposal, here?

Kind regards,
K1LLUM1N471

I followed your suggestion and instead of deleting all 20 entries of type = '3', I reduced them finally to 6 entries by the help of the debug information of the daily build version.

a:7:{i:10;s:81:"20:Rückmeldung,30:anerkannt,40:bestätigt,50:zugewiesen,80:erledigt,90:geschlossen";i:20;s:73:"10:neu,30:anerkannt,40:bestätigt,50:zugewiesen,80:erledigt,90:geschlossen";i:30;s:75:"10:neu,20:Rückmeldung,40:bestätigt,50:zugewiesen,80:erledigt,90:geschlossen";i:40;s:75:"10:neu,20:Rückmeldung,30:anerkannt,50:zugewiesen,80:erledigt,90:geschlossen";i:50;s:74:"10:neu,20:Rückmeldung,30:anerkannt,40:bestätigt,80:erledigt,90:geschlossen";i:80;s:76:"10:neu,20:Rückmeldung,30:anerkannt,40:bestätigt,50:zugewiesen,90:geschlossen";i:90;s:73:"10:neu,20:Rückmeldung,30:anerkannt,40:bestätigt,50:zugewiesen,80:erledigt";}
mysql> DELETE FROM mantis_config_table WHERE (config_id = 'status_enum_workflow' AND project_id = '0' AND type = '3');

{"reporter":1,"handler":1,"threshold_min":100,"threshold_max":0}
mysql> DELETE FROM mantis_config_table WHERE (config_id = 'default_notify_flags' AND project_id = '0' AND type = '3');

{"owner":{"monitor":1,"bugnotes":1},"reopened":{"monitor":1,"bugnotes":1},"deleted":{"monitor":1,"bugnotes":1},"bugnote":{"monitor":1,"bugnotes":1},"relationship":{"monitor":1,"bugnotes":1},"feedback":{"monitor":1,"bugnotes":1},"acknowledged":{"monitor":1,"bugnotes":1},"confirmed":{"monitor":1,"bugnotes":1},"assigned":{"monitor":1,"bugnotes":1},"resolved":{"monitor":1,"bugnotes":1},"closed":{"monitor":1,"bugnotes":1}}
mysql> DELETE FROM mantis_config_table WHERE (config_id = 'notify_flags' AND project_id = '0' AND type = '3');

{"owner":{"monitor":1,"bugnotes":1},"reopened":{"monitor":1,"bugnotes":1},"deleted":{"monitor":1,"bugnotes":1},"bugnote":{"monitor":1,"bugnotes":1},"relationship":{"monitor":1,"bugnotes":1},"feedback":{"monitor":1,"bugnotes":1},"acknowledged":{"monitor":1,"bugnotes":1},"confirmed":{"monitor":1,"bugnotes":1},"assigned":{"monitor":1,"bugnotes":1},"resolved":{"monitor":1,"bugnotes":1},"closed":{"monitor":1,"bugnotes":1}}
mysql> DELETE FROM mantis_config_table WHERE (config_id = 'notify_flags' AND project_id = '10' AND type = '3');

{"owner":{"monitor":1,"bugnotes":1},"reopened":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"deleted":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"bugnote":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"relationship":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"new":{"threshold_min":"70","threshold_max":"70"},"feedback":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"acknowledged":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"confirmed":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"assigned":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"resolved":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"closed":{"monitor":1,"bugnotes":1}}
mysql> DELETE FROM mantis_config_table WHERE (config_id = 'notify_flags' AND project_id = '4' AND type = '3');

{"owner":{"monitor":1,"bugnotes":1},"reopened":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"deleted":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"bugnote":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"relationship":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"feedback":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"acknowledged":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"confirmed":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"assigned":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"resolved":{"monitor":1,"bugnotes":1,"threshold_min":"70","threshold_max":"70"},"closed":{"monitor":1,"bugnotes":1}}
mysql> DELETE FROM mantis_config_table WHERE (config_id = 'notify_flags' AND project_id = '5' AND type = '3');

Kind regards,
K1LLUM1N471