For a long time I’ve had the error message “failed while running mythtranscode to cut commercials” on my MythTV box. Today I did some searching to find out more about it.
It turns out the error is caused by the existence or permissions of .ICEauthority.
I nuked ~/.ICEauthority and was able to mythtranscode. Apparently this file is recreated occasionally, so if I see this again I’ll check permissions.