Actually, I had same problems in SVP 12 for awhile, then everything became solved somehow.
Now the same problem is with SVP 13 x64.
BUT 2 days ago after installing xvid, I rendered in xvid avi without any issues, then the problem appeared again.
So, it’s not true that SVP can’t accept xvid on its timeline or can’t use it for render!
Right now I remember that there is a xvid version from 2009 which works with no any problems, I’ll try to find it and then I will write the result here.
The question is about Sony AVC/MVC – I read already in a forum about the same general problem especially with SVP 13.