After the do-release-upgrade switched me from the defunct Mythbuntu distro to Ubuntu, and clobbered a bunch of my MythTV packages in the process, I've nearly got everything working again.
I had to reinstall some mythtv-* packages (notably mythtv-database), and set up MySQL after the upgrade removed it. Luckily all the data was intact. In MySQL, I had to enable networking and grant permissions to the mythconverg database. Now I can successfully connect to MySQL on the MythTV master backend server from the client, so the database connection is OK. Also, I'm able to run the MythTV frontend locally on the master backend server, so I know the MythTV stack there is working.
However, both the dedicated MythTV frontend as well as my standard Ubuntu workstation (which did not go through this upgrade) tell me "Connection refused" on the MythTV port 6543 when trying to connect to the master backend. Nothing shows in syslog on the server side, and 6543 is in use ("nc -lp 6543" fails). The UFW firewall is inactive/disabled on client and server (and workstation). This was not a problem before the upgrade, so it's extremely unlikely network hardware is to blame.
I'm out of ideas to figure out what is blocking the traffic to the MythTV backend master server. Does anyone else have suggestions?
Additional note: Before I fixed the MySQL access, the MythTV Frontend initially would either start without any existing config or just crash. Once I got the MySQL access fixed, both the Frontend machine and my workstation started up with the unique MythTV themes I had chosen for each before the upgrade. This seems to indicate the MythTV Frontend connects to SQL just fine, it's just the MythTV backend traffic on port 6543 that seems to be failing.
2017-03-12 Update: Running
netstat -4lnp |grep :6543 returns:
tcp 0 0 127.0.0.1:6543 0.0.0.0:* LISTEN -
So this makes me think the port is binding to the 127.0.0.1 address rather than the actual server address I've entered. Even though the local Myth frontend on the server points to the full server address, this could explain things, but I'm not sure how to fix it even if this were the case.