"Error sending to Soundtrack Pro"

I was trying to send a clip to STP as a multitrack project, and I got an error message - my scratch disk was out of space. So I erased some unrelated files and reclaimed about 150 GB. After a restart, I can send clips to STP as an audio file, no problems there, but when I try to send as a multitrack project I still get the "Error sending to Soundtrack Pro" message and "General Error". I get the same errors if I try the process with other clips in the same sequence. I tried duplicating the sequence and got the same errors, and the same thing happens with all other sequences in the project.

My film is about 80% done and I'm getting a little worried.

I'm using the latest version of Final Cut Studio.

Any suggestions would be greatly appreciated.