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I just realized, I had nearly forgotten to follow up on this. Yes: it  
turned out I had a source of stray light in the dark box surrounding  
the scope: the glow in the dark tags used to label the filter wheel  
positions on the original Olympus body. Removing them gave me a major  
improvement, blackening several polished (and thus reflecting)  
surfaces in the detection path did the rest.

Just in case someone stumbles over similar problems.

Thanks for all helpful hints,

Christian

> My thought, however, was more along the lines of stray light. With
> wide-field collection you have to be  absolutely sure that room light
> (even from the LEDs on some equipment in an otherwise dark room) is
> not entering the objective or reaching your PMTs through gaps in the
> light guides. Is there some what you can wrap the PMT in black tape
> just for comparison?