This is essentially the same problem described in:
http://forums.rockbox.org/index.php/topic,51153.0.htmlExternally-generated playlists (e.g. from MediaMonkey, gPodder) were playing for me just fine, until I upgraded to a nightly build some time in the middle of May. I just tried updating again today, with the same problem.
My config file looks like this:
# .cfg file created by rockbox 00a3658-160517 - http://www.rockbox.org
volume: -21
idle poweroff: 1
start directory: /<microSD1>
autoresume enable: on
eq enabled: on
dircache: off
fmr: /.rockbox/fmpresets/preset01.fmr
start in screen: wps
playlist catalog directory: /<microSD1>/Playlists
Semitone pitch change: on
And an example playlist (located in `/<microSD1>/Playlists`) would be:
I:\Music\Boards Of Canada - Twoism\01 Boards Of Canada - Sixtyniner.mp3
I:\Music\Boards Of Canada - Twoism\02 Boards Of Canada - Oirectine.mp3
I:\Music\Boards Of Canada - Twoism\03 Boards Of Canada - Iced Cooly.mp3
I:\Music\Boards Of Canada - Twoism\04 Boards Of Canada - Basefree.mp3
I:\Music\Boards Of Canada - Twoism\05 Boards Of Canada - Twoism.mp3
I:\Music\Boards Of Canada - Twoism\06 Boards Of Canada - Seeya Later.mp3
I:\Music\Boards Of Canada - Twoism\07 Boards Of Canada - Melissa Juice.mp3
I:\Music\Boards Of Canada - Twoism\08 Boards Of Canada - Smokes Quantity.mp3
From past behavior, Rockbox should be smart enough to strip off the `I:\` and start looking for the files in `/<microSD1>` instead.