UTF-8 encoding are the problem here. Most likely you copied the files from a windows system (using ISO-8859-1 encoding) to a linux system (using UTF-8). A computer can only store bits. The mapping of a combination of bits to a real character a human can read is called the encoding. E.g. using UTF-8 the letter
a is encoded as
01100001. For umlauts those bit sequences differ in the two encodings, so a UTF-8 system displays another character or doesn't know the bit sequence alltogether. That's the whole problem.
With the shell program convmv /
sudo apt-get install convmv you can rename files with broken umlauts to their correct UTF-8 charset. Usage:
convmv --notest -r -f ISO8859-1 -t UTF-8 <Verzeichnisname
Run that on your files after installing
convmv by clicking on the Software Center icon above or using the shell command provided next to it. Then try syncing again.
Good luck on sharing German history! :D