As reported by Developer58 and confirmed by kiwijunglist, mbuzina, HomeY and hwahrmann:

When flac files contain a null character SQL fails to insert the track into the database

h4. Steps to Reproduce
Tag music using a tag editor that does not strip Null characters, such as J.River media center.
Try to import the track into MP

h4. Additional Information
Merged to Master https://github.com/MediaPortal/MediaPortal-1/commit/4e5705643b62f3785beb0d124e75f00302addaaa