Temporarily no sound after turning on

My UE megaboom had worked fine until now, but since last week it has developed this weird problem: There is no sound for about 5 minutes or so after turning it on.

Pairing works fine and I can play music immediately from my device, but there is no sound, not even the startup sound from the speaker. After about 5 minutes the sound will suddenly start (sometimes with a little crackling). It then works fine for as long as I use it.

I tried a factory reset, re-pairing but no change.

Anyone have any ideas about this?