I feel like I can contribute something worthwhile after asking many questions.
Today I got the dreaded error message "Error compiling movie. Unknown error."
Maddening.
After trying to figure out which step in my editing caused this, and being unable to reverse it, I created a new sequence with identical settings to the one that I could not render and then copied and pasted the clips from the old sequence to the new.
It works fine.
So clearly something has gone screwy with the sequence.
So if you get the warning dialogue, see if this works. I hadn't seen this solution on the boards anywhere.
Error compiling movie. Unknown error is a big pain in the but.
Creating a new sequence does not work for everyone.
You have to try all sorts of things to narrow the problem down.
I once had that error and after messing about a whole afternoon i found a falty clip.
Anyway thanks for reporting your solution.