Nextcloud 19.03snap2 - login textboxes missing

Hi community,

I am running a Nextcloud 19.03snap2 installation on Ubuntu 20.04LTS. More than one month ago I noticed that I could not access the apps page of the administrator account anymore. Some days later the desktop sync tool stopped syncing, now login textboxes on the login screens are not there anymore. My custome theme is still visible, but the the login text boxes for username and password are missing. So I cannot access my cloud anymore.

At the moment I have no clue where to start looking for the problem. I have checked the snap logfiles

Logs for Nextcloud can be found at: /var/snap/nextcloud/common/nextcloud/data/nextcloud.log
Logs for Apache can be found at: /var/snap/nextcloud/current/apache/logs/
Logs for Redis can be found at: /var/snap/nextcloud/current/redis/redis.log
Logs for PHP-FPM can be found at: /var/snap/nextcloud/current/php/php-fpm.log
Logs for Certbot can be found at: /var/snap/nextcloud/current/certs/certbot/logs/letsencrypt.log
Logs for MySQL can be found at: /var/snap/nextcloud/current/mysql/localhost.localdomain.err

but I cannot see any clear indicator. What to look for? What could be the root cause?

The disk of the server is not full, some memory is still free, a reboot of the (virtualized sever) does not change anything.

Where to start? Any hints for me?

Many thanks in advance