"Forbidden You don't have permission to access /loleaflet/2.0.4/loleaflet.html on this server"

Hello everyone,

I have Nextcloud 11.0.2 running on an Ubuntu 16.04 server (xxx.de) and want to integrate Collabora. I have Collabora setup on the same server (office.xxx.de) and it seems to be running without any issues from what I can tell. I have enabled the Collabora app in Nextcloud and added the office.xxx.de-URL in the settings. However, when trying to open a file, I get the following error message:

Forbidden

You don’t have permission to access /loleaflet/2.0.4/loleaflet.html on this server.

Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.
Apache/2.4.18 (Ubuntu) Server at office.dsincloud.de Port 443

Any ideas what could be causing this? I have a hunch that it is related to the fact that the server is using ISPConfig to set up sites and every site has its own user. If so, how do I give the correct permissions to the Nextcloud-site-user to allow access to Collabora?

Any help would be highly appreciated!

Cheers