You can use 'strace' and the direct commandline to help figure out where the problem might be. Run this:
strace mencoder (myvideo.avi) -o test_bb.avi" -of avi -ovc lavc -oac lavc -lavcopts vcodec=mpeg4:vbitrate=230:acodec=mp3:abitrate=64 -vf scale=240:135
Replace (myvideo.avi) with your actual video, of course.
There will be gobs and gobs of output, but somewhere towards the end and the SIGSEGV (seg fault) should be something that looks interesting; it could be trying to open a file/directory and getting denied, calling a function that's getting an unexpected result, who knows. It'll be the first clue, though - what is making mencoder barf on your system?