Thanks for your pointers! I finally managed to migrate to nextcloud using these steps:
This is pretty sketchy, because I disabled several safety checks, but in case someone wants to do it:
- Manual upgrade according to https://docs.nextcloud.com/server/17/admin_manual/maintenance/migrating_owncloud.html
- Manually allow php 7.3 (index.php and lib/versioncheck.php) in nextcloud 12.0.13
- Force upgrade between major versions (my initial issue) in lib/private/Updater.php by removing the line throw new \Exception('Updates between multiple major…
- Follow https://github.com/nextcloud/server/issues/14549 to fix the error that occurs
- UPDATE oc_filecache set mtime=0
- UPDATE oc_filecache set storage_mtime=0
- /3rdparty/pear/archive_tar/Archive/Tar.php:639 remove “&” in front of func_get_args() (https://www.dotkernel.com/php-troubleshooting/fix-installing-pear-packages-with-php-7-2/)
- run updater from settings (remove php check again (only until nexcloud 14 is installed)) until newest version is installed
After doing this I noticed that some files have last edited dates in the future. This is probably due to setting mtime to 0.