Perhaps you have non-standard (ie accented like ø) in the Collection name? Although in Norwegian it would seem to be musikk.
It would seem to be a bug on Popcornhour's behalf if it only accepts Music. In any case you can create a debug log (step 4b) of Popcornhour accessing MediaMonkey's Media Server and send it to support: viewtopic.php?f=7&t=69
Maybe the developers can see a way to fix this. I would still suggest contacting Popcornhour as well about this issue.