Hi Steffen,
Unfortunately TG2 only supports basic ASCII English characters in names and file paths etc. That's basically A to Z, 0 to 9 and the basic English punctuation and special characters. Accented characters are not supported.
You can fix your files by opening them in a text editor and changing or removing the accented characters. Your filenames won't be pointing where you want them but at least the files will open in TG2 again.
Regards,
Jo