@User12345 You may have been hit by a problem that is plaguing others as well.
Which versions of NextCloud and Collabora are you running?
Please run the following two commands on the server running the Collabora container and post the response (sans your real domain/ips of course):
curl -k https://localhost:9980/hosting/capabilities
curl -k https://collabora.mydomain.com/hosting/capabilities
The outputs should look like this:
{"convert-to":{"available":true},"hasMobileSupport":true,"hasTemplateSaveAs":true,"productName":"Collabora Online Development Edition"}
If the second command’s output has {“available”:false} in it, you should follow the thread I linked to above and wait for a solution. Frustrating as hell, but as of now I have no idea where exactly the issue lies.