Thanks a lot for your engagement. I could easily provoke the issue on any lens by forcing short exposure times. The critical time is around 1/2000s, that is when some frames start to appear black. Firing a sequence of pictures of same subject in same light shows some improvement, the trend is that subsequent frames are generally more exposed that previous ones.
At 1/4000 all frames are black.
At 1/1000 some frames are acceptable and can be rescued (-1EV?)- All frames are underexposed, some really badly (-2EV?)
Camera is now sent back to Olympus Service with a guarantee claim.