szurkebarat,
I recently discovered that PM (ver 4.6.1 XP) has some troubble using ü and ű ancient letters in the filenames. In fact PM doesn't allow to use them at all. When renaming files with pressing enter on a thumb in the contact sheet I cannot even enter this two letters. It might have been an OS restriction but it is not as I can easily rename the files using either of the letters in Windows Explorer or any other file manager, and PM even displays and handles the files renamed by the OS just it won't do the same for me.
How would I enter those characters on my system so I can test?
-Kirk
I don't have a complete answer for it. I use a Hungarian keyboard set. If you can switch your keyboard layout to it then you find this to characters mapped to _ and \ keys.
You can use Alt codes as
Alt gr+129 for ü
and
Alt gr+154 for Ü
For the other ancients (ű and Ű) I didn't manage to find the Alt code. I use the same keyboard layouts on my Mac and there is no problem with the anciens in PM during renaming. I don't know if this problem exists only with Hungarian characters or german, norvegian and other users having ancients do have similar issues.
I hope this helps you solve the issue.