Hi Søren,
could it be that your Mattermost domain also supports IPv6 (i.e. it has an AAAA record in the DNS)? If so, calls is not supported on IPv6 currently, so the ICEHostOverride setting would either need to be set to the IPv4 address of your server or to a v4 only domain name for the time being.
Also when you stop the calls plugin, can you then confirm using nc that you’re able to communicate on the port 8443 with the server?
You can run the following command on the server:
nc -l -u -p 8443
and then you run this command on a different server (or your client):
nc -u <the-name-in-the-icehostoverride> 8443
You can then start typing in any of the two windows, the output should be mirrored to the other side after you hit return - if not, the connection is having issues and needs to be checked.
If that works we need to dig deeper.