I had the bug "cannot setup codec, bitrate too low". It turned out that this wasn't the problem. The problem was that I had german umlauts in the filename, e. g. "Schnäppchen". When I changed the filename everything worked fine.
It would be great if someone could fix this wrong error message or better yet fix avidemux so that this great software can also deal with filenames beyond standard ascii character set.
While I'm at it: Thanks for this very useful software! :)
Add-On: I just figured out that the target filename is to blame, not the source filename.