Hello Timo,
You are correct, and I just got this all sorted out earlier today. As to format, it was the standard unix mbox format. As to what I was renaming, I have a subdir in my mail folder area for maillists, that procmail feeds into, with a separate file for each list (like this one), and though most files were mode 600, a few were like 644, and I guess it must look at the overall directory or something permissions wise. I went in a little earlier and changed ALL files to mode 600 in the folder, and now it is happy to rename things.
In the end, issue solved, and apparently you are right in the newer 2.1 is more picky, as I have done this yearly rename for the past couple years without incident, but this year it blew up on me. Again, many thanks for the response..
Howard Leadmon
What mailbox format? What are you renaming and where? What are those directories' permissions? Anyway newer Dovecot versions are a bit more strict about directory permissions being same in source and destination.