it’s not really wrong and it is plain http://. this internal URL is AiO voodoo to avoid internet loop.. kind of ugly splitbraindns workaround.. see Local access for Collabora and referenced Github issue for details..
but thank you pointing my attention to this URL. I think the problem is you have wrong value configured for CODE to access Nextcloud
which results in wrong document URL:
From my understanding it must be
| source | destination |
|---|---|
| Nextcloud | nextcloud-aio-apache:23973 |
| CODE | nextcloud-aio-apache:{APACHE_PORT} (by default :11000) |
in the Github there is some discussion about similar problem