OK, I thought you already downgraded.
You need to downgrade to nethserver-nextcloud-1.16.7-1 and then go step by step up to 1.19.7-1.
Don’t downgrade. Downgrading did not work for me, I got an internal server error.
This method worked for me:
I updated to latest nethserver-nextcloud and followed
b) Migration scripts to latest version (experimental procedure):
from the wiki.