thanks for all your replies guys. The problem was in the MXF, not in the original file. Batch importing didn’t work but deleting and re-importing the infected MXF solved the problem. Not sure why this made the difference, maybe because batch importing means that Avid rewrites certain components of the media while keeping other settings from the old media.
As to the big question of OMF vs AAF, you guys are probably right. But if the sound guy prefers this I won’t argue with him. As long as he does his job and delivers the sound to everyone’s satisfaction. If it is technically possible to create an OMF, I will do that for him. In my case, if the OMF hadn’t worked after the troubleshooting, I would probably have convinced him to accept an AAF. But luckily I didn’t have to.