I recently switched from a Mandrake-based 0.16 configuration to R5A16. In the process I copied over my nuv/png files and did a mysqldump/import. When I tried transcoding some of my old files with nuvexport I saw the following:

Waiting for mythtranscode to set up the fifos.
Waiting for mythtranscode to set up the fifos.
Waiting for mythtranscode to set up the fifos.
...
Waited too long for mythtranscode to create its fifos. Please try again.

I tried transcoding using the job options on the Manage Recordings screen. The job showed up in the queue, but failed as "errored". The logs said:

2005-07-01 00:36:22.672 Attempted to transcode myth://192.168.1.5:6543/1085_2005011802 ... 023000.nuv. Mythtranscode is currently unable to transcode remote files.

I made sure the backend was configured to allow transcoding of files recorded on a different backend in mythtv-setup. Still no luck.

From some Googling I learned that mythtranscode checks the 'recorded' table to see where files were originally recorded. Turns out my Mandrake hostname was different from KnoppMyth's "mythtv". The fix was to manually update the table with mysql:

UPDATE recorded SET hostname = 'mythtv' WHERE hostname = 'oldname';

Seems to work so far. Hope this helps folks in this unusual situation.