I just installed 9.1 over 9.0. I have a lot of learning to do on it yet (if anyone thought 9.0 was bewildering, wait until they see 9.1!), but I've ripped some CD's and rebuilt a couple of custom views. But there is one thing that hasn't worked for me yet. Normally when I rip a CD whose info isn't in YADB, I decline the dialog box, fill in the info with the usual tag editor, and then right-click on the CD and pick "Submit to CD database" to share the fruits of my labor.
When I do this in 9.1, it immediately responds with an error pop-up "Failed to submit disk Data CD to CD database".
Odd message -- it's not a Data CD.
There is a slight wrinkle -- I had first played this CD (without filling in any info) in 9.0, so it remembered that it was "unknown cd (4)" or some such when I inserted the disk this time. Some library incompatibility? Next time I do an obscure (but not before seen by 9.0) CD, I'll have to see if works any better.