This has happened twice so far.
I have been cleaning up and merging some year specific directories.
1)Change Directory to e.g. "More 1969"
2)Clean up tags in this directory
3)Add Directory e.g. "1969"
4)Sort by Title
5)Ctrl Select duplicates in "More 1969"
6)Ctrl Delete all duplicates
7)Ctrl-D to "More 1969"
MP3Tag goes into permanent wait (hour-glass) at this point without consuming CPU.
Have to Close Process (Shows as Stopped Responding)
No problem repeating step 7 after re-starting MP3Tag.
The bug seems to be new to 2.35J.
I left MP3Tag running over-night the second time. Wait never cleared.