Browser asks for login on Overview Tag

I did a fresh installation of Nextcloud on a fresh Apache instance.
Whenever I go to settings > overview the Browser shows a Popup asking for Login credentials. The Overview tab is the only page that shows the popup. I can click away the popup and the page is shown without issues.

Nextcloud version (eg, 12.0.2): 18.0.3
Operating system and version (eg, Ubuntu 17.04): Synology NAS (Linux)
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4
PHP version (eg, 7.1): PHP 7.3

The issue you are facing:

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

Steps to replicate it:

  1. Login into the web interface.
  2. Go to Settings > Overview.

The output of your Nextcloud log in Admin > Logging: There are no corresponding log entries.

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

$CONFIG = array (
  'instanceid' => 'xxx',
  'passwordsalt' => 'xxx',
  'secret' => 'xxx',
  'trusted_domains' => 
  array (
    0 => 'xxx',
  'datadirectory' => '/volume1/NextcloudData',
  'dbtype' => 'mysql',
  'version' => '',
  'overwrite.cli.url' => 'xxx',
  'dbname' => 'nextcloud',
  'dbhost' => '',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'nextcloud',
  'dbpassword' => 'xxx',
  'installed' => true,
  'memcache.local' => '\\OC\\Memcache\\APCu',

The output of your Apache/nginx/system log in /var/log/____:
No entries in the apache error log in that timespan.

Can you show us a screenshot?
Maybe Synology webserver setup do something?
Are you running a VM or using Synology OS?

I tested a bit further and observed following:
I still had the warnings regarding well-known/caldav and well-known/carddav.
I added the Rewrite Rules to several config files. The carddav one is gone, the caldav one is still there.
I added the Rewrite Rule to more configs/.htacces files and rebooted the server.
The caldav message is gone and I don’t get the login popup.
After a while I reload the page, the message is again there and so is the popup.

Maybe this narrows things down. If needed I can post related configs.
I directly run Synology OS.

EDIT: OK, it seems to be the other way round. If I enter the credentials of the NAS Admin user the caldav message is gone, if I cancle the login, the message is there again.
Did I miss some access rights?