Acces denied after adding wordpress

Ubuntu 18.04

I was running Nextloud 18 for some time without any issues on a subdomain. There was a placehoder at the root-domain.

After adding a wordpress installation (on the root domain) i could no longer connect to nextcloud, on the web client the only thing reported is blank page with access denied.

As I was not able to get the subdomain working I moved Nexcloud to a subfolder on the WP installation (/var/www/wordpress/nextcloud).

Once that did not change anything, I decided to remove nextcloud and reinstall version 21 (in the subfolder of wordpress).
That did look to go very well. I could finish the installation on the webinterface to connect to the existing database and data-path. Happy, as there is a lot of data on Nexcloud (dont worry, there is a backup).

However, after the install finalized and I would be sent to the dasboard… the blank page with acces denied returned.

I am kind of lost at this moment as I checked ownership, and anything else i can think of.
Anyone recognizes this and can lend a helping hand?