Hi BetterSense. I tried testing the shutter with the TV-method. I found it hard to diagnose, they looked more-or-less ok, but it's hard to see precisely...

John, thanks for the informative reply! During the last CLA, oily magnets were cleaned (I had a problem with a way too long shutter time on the first exposure of the day back then).
I use APX-100 in rodinal 1+50 for all my cameras since a month or 2. At first all cameras were giving OK exposure (my notes of 6 weeks ago when I did some testing say: Zone I = 0.25 (base+fog=0.15), zone V = 0.75, zone VIII = 1.35, zone X = 2.05), but since 2 weeks only the OM-2n gives overexposure (all zones give a higher density), even though I kept using the same film/dev-combination...I just tried two other types of film and another developer to be sure that that was not the problem.
I just bought some fresh silver oxide cells today (v76px, same as the old cells). Even though the old cells are relatively new (4-5 months), their expiry date was sep/2009 and I used the camera quite intensely (about 3 rolls/week). New cells have expiry date nov/2011. I will test the camera tomorrow (I won't look at the exposure needle for now, just put in the aperture/shutter speed I read from my light meter).