and the curl on https://collabora…eu/hosting/ is also responding on port 443? If you are on local and you use port 9980, do you change this port to 443 (or is this proxied again?).
If yes, can you try it without a proxy (not sure if collabora does https on its own)?
the problem is that the charts on truenas do not support ipv6 name resolution. But nextcloud.mydomain.com only has an ipv6 aaaa record. The result was that the collabora instance is not able to access files on nextcloud.mydomain.com.
The workaround for this is to use the kubernetes coredns pod to resolve the ipv4 addresses for these domains. after adding some lines in the coredns configmap with: