Okay, so I have never used Myth (yet) but did read something about it
a few months ago. So don't count me as an expect, & forgive me if you
know this bit...

Myth needs both a "back end" and a "front end" to work. These can be
on the same PC or separate ones. The back-end runs a database, does
the recording, etc. The front-end then connects to the back-end and
runs the presentation layer to show the recordings & user interface.
The idea is you can have a network with one BE & several FE if you want.

The messages "No UPNP backends," "Unable to login to database," etc.
sound as if you only have a FE & no BE. Alternatively you could have
connectivity issues from FE to BE. Maybe the BE is crashing, not
running, etc.