I have found the right reason of why “Could not establish connection to the Collabora Online server” in my server, however don’t know how to solve it.
That is, it seems that can’t set ports to others instead of 443 in richdocument 3.6.0.
In my WAN network, port 443 is shielded. So that I have to set a port forwarding rule in my router:
External port: 54321, Internal Port: 443, Protocol: tcp , Internal Ip: Ip of my centos server
and then using the External port ( https://office.domain.name:54321 ) to vist my Collabora Online server form internet. It works fine in richdocument 3.5.3.
I just test it in the LAN network under my router. And set the “URL (and Port) of Collabora Online-server” to https://office.domain.name:443 or https://office.domain.name, then it says: Collabora Online server is reachable. However, Unreachable for https://office.domain.name:54321, and at the same time in my a nother nextcloud test server installed with richdocument 3.5.3, the https://office.domain.name:54321 still works fine.
So I think I have found this bug.
Any suggestion to solve this?