Undo Convert | Tag - Filename fails to undo folder creation, i.e. the Undo doesn't remove the folders (if any) that were created by the Convert.
Also, I suggest Convert | Tag - Filename should (optionally and by default) delete any folders it empties. Currently if it changes e.g. oldfol\file.mp3 to newfol\file.mp3 and oldfol contained no other files, it leaves oldfol .
Empty directories are now removed at undo Convert > Tag - Filename with the current Development Build.
Not here (V2.39f), Florian. I.e.
Format string: ..\newer4\file
Preview: "D:\newer3\file.wma" -> "D:\newer4\file.wma"
leaves newer4... and newer3!
When I'm applying the converter Tag - Filename on the file
E:\Mp3tag\Test\test.mp3using the format string
..\New\testa new directory
E:\Mp3tag\New\gets created and the file test.mp3 is moved there. When I choose 
Undo, the directory
gets removed again. Sorry, but it seems to be working fine here.
(Oops, I overlooked your word undo and thought the fix was for the other issue. Serves me right for conjoining reports. Sorry to waste your time Florian. )
Confirmed in V2.39f. Thanks
Confirmed in V2.39f.
... on single files. But on multiple files going to the same directory, undo gives the error
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. Directory c:\mp3t\ cannot be deleted.
.
. » Please check the drive and make sure that no other
. processes are using the directory.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
even though the operation succeeds.
Oddly, this happens even when only one of the selected files actually got renamed e.g. if the others failed due to "file exists." Suggesting there may also be a flaw in the undo buffer insert.
PS Perhaps that "»" is a typo in the Mp3tag dialog.