What app is this about, and its version: Collabora 25.04.10.3~ynh1
What YunoHost version are you running: 12.1.40.1 (stable)
What type of hardware are you using: VPS bought online
On my Yunohost server I have two coolwsd.xml files
a) /etc/yunohost/apps/collabora/conf/coolwsd.xml
b) /etc/coolwsd/coolwsd.xml
How can I know which one is the correct one?
Should I delete one of them?
Note: It seems that my Nextcloud instance writes to b)
I can see, that in b) my nextcloud instance and the password I set for the collabora server’s web interface are correct. Still, that password does not work.
I have now added the correct password to a) as well. I still cannot log in to https://cb.gnubahn.dk/browser/dist/admin/admin.html
BTW: The server works fine - I can open and edit documents from my Nextcloud instance. The correct URL for my nextcloud instance is only in b).
This one is the template file.
What does that mean?
This one isn’t used?
If so, no wonder nothing has worked! That is the one I have been using all the time to make changes.
This is the file from collabora_ynh/conf/coolwsd.xml at master · YunoHost-Apps/collabora_ynh · GitHub which is copied and then filled in with the variables specific to your installation collabora_ynh/scripts/install at 620d836e7cd570a5c44f5e870e19a22fa42c0494 · YunoHost-Apps/collabora_ynh · GitHub.
Does it contain any special characters, quotes …?
No only numbers and letters (different cases)
I see, thanks.
So the code you showed me about how to add groups in order to make several sub.domains work should be pasted in this one. It is only the part between <alias_groups mode="groups"> and </alias_groups>, right?
Yes, it is.
Do you believe it would help to change the password in the coolwsd.xml file and then try again?
I think so. Just restart the coolwsd service after.
I still cannot acces the web interface - it seems that the password is not accepted, but I get no message about something being wrong with password or username.
BUT. now I can use my sub.domain to open and edit files fro my Nextcloud instance.
HOORAY. Great F(explicits)ng thank you so bloody fantastically super much 
Same when trying in a private tab or after clearing cookies ?
Yes