After upgrading to 15.0.11, the app store/section can not be opened

Nextcloud version (eg, 12.0.2): 15.0.11
Operating system and version (eg, Ubuntu 17.04): raspbian stretch
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.25
PHP version (eg, 7.1): 7.0.33

The issue you are facing:
After upgrading from 15.0.8 to 15.0.11, I got the message, that twofactor-auth is going to be disabled (like the other upgrades before, this is normal).
When trying to go to the Apps section to re-enable twofactor-auth, the normal app section is not appearing, but the URL is jumping from
back to

and the browser is only showing a nearly white page (except the nextcloud header).

Is this the first time you’ve seen this error? (Y/N): Yes

Steps to replicate it:

  1. follow the instructions mentioned above

The output of your Nextcloud log in Admin > Logging:

Error	core	1 two-factor auth providers failed to load	

The output of your Apache/nginx/system log in /var/log/____:

tail -f /var/log/apache2/error.log:
[Wed Sep 11 17:03:20.689356 2019] [authz_core:error] [pid 27751] [client] AH01630: client denied by server configuration: /var/www/html/nextcloud/data/.ocdata

Hope, this is only a fixable error…
Thank you very much in advance for any help.