I've tried running this down on the Nextcloud forums as well. I have installed Collabora and Nextcloud Office on my Nextcloud instance, and while I can create, open and edit documents on the Nextcloud test site, when I try it on my own Opalstack install, documents "stall out" upon creation / open.
The creation dialogue takes me through the create/name/choose a template dialogue, but then it's a "forever circle" where the opening of the document never completes. The same "forever circle" when I try to open an existing document (including but not limited to the documents created through the above process).
Wondering if this is something I need to deal with on this hosting end instead of an issue with Nextcloud itself. I'm very low-level in tech expertise, but when I try to look at the console it seems to return tons of "sync" and "notification" commands... screenshot at https://imgur.com/a/lbG8Dub.