Thank you jeam25, I did everything you listed, but to no positive effect :/
I figured out it could be something with the MIDI files I already mentioned, and in fact the problem seemed to appear only in the later versions of the session with the MIDI already somewhat edited.
So I went back to one of the first versions, where everything seemed OK, and imported just the audio tracks and settings from the latest one, except for the MIDI. Similar solution to what TOM@METRO suggested (thanks!).
It looked like it's fine for now, I worked for half an hour or so with no interruptions, and then suddenly it appeared again! Not the MIDI issue this time, since I did not follow the steps I had taken before with the original faulty session.
I wrote to Avid support yesterday, but I guess if "Error 6" is still such a problem for many users, and has been for some time now, it may be impossible to solve once and for all.
It would be understandable if such thing happened with a free and unsupported DAW, but Pro Tools? The best, expensive, one and only "industry standard?" Oh, come on!
