A few days ago last.fm changed their 1.2 [url=http://www.lastfm.de/api/submissions]API[/url] making the MusicBrainz parameter mandatory now (even if unknown).

In MediaPortal.log you could read something similar to this:
"[ERROR][Scrobbler submit]: AudioscobblerBase: Submit failed."
Or with debug logging enabled:
"FAILED Plugin bug: Not all request variables are set - got 17 parameters."