Perhaps you can try to execute the occ command.
It should be something like this in FreeBSD “su -m www php /path/to/nextcloud/occ” and use the “maintenance:repair” and “integrity:check-core” to see if the nextcloud files got a problem.
Its only a guess
I already tried that 
As well as:
su -m www -c "/usr/local/bin/php -f /usr/local/www/nextcloud/occ integrity:check-app serverinfo"
Seems like I am not the only one.
This issue should get fixed with NC v20.0.1.