@tim_ba: Thank you for suggestion. I changed title of the issue, as well as added some edits to first post. Un fortunately, I had to hurry because of timing, and decided to migrate to v7 with fresh install, moving accounts with imapsync. For me and my timeframe this was the only option. We are done for 90% for now. Due to this, we will switch off our old VMs in a nearest month, and I have no further options to deal with migration issues. I'll leave this post open, if in future somebody can find similar issues or solutions, if nobody mind that.