My OM-2n (which has been very happy for the last six months or so after CLA) just developed a new problem: it overexposes every shot with about 1.5 stops. I checked all the obvious and have the feeling I'm missing something very easy/stupid....

-Batteries are 4 months old and give 3.1V, battery terminals OK
-Lightmeter of om-2n is completely the same as my handheld calibrated meter (in both auto-mode and manual-mode).
-Overexposure happens both with open and closed aperture (same density for zone V exposure @ f/1.4-1/1000 or f/22-1/4). it happens with all my 3 zuiko lenses.
-Happened on 3 different brands of film in 2 different types of developer
-Developed a roll of APX-100 from my om-2n together with a roll of APX-100 from my canonet-28 in 1 tank: roll from canonet was perfect, from om-2n overexposed by about 1.5 stops.
-No exposure compensations set.
-No difference between auto-mode and manual-mode. In auto-mode a long exposure (5 seconds or so) gives exactly the same overexposure.

What am I missing???