Auxiliary data is truncated if it contains special characters (although the encoding and the corresponding setting in config.xml is right).
I found that the truncation depends on how many special characters are contained in the string.
E.g. "Björk" --> "Björ" (1 character truncated)
but "José Gonzáles" --> "José Gonzál" (2 characters truncated)
UTF-8 works in a way that it uses an additional byte for a "special" character like an umlaut. For me it looks like there is some bug in the code which mixes up the number of "human-readable"-characters for a string (which would be 5 for "Björk") and the number of bytes actually needed to store it in UTF-8 (which would be 6 in that example).
For the record: I used the newest SVN-version in combination with taglib.