I don't think that error is causing mencoder to exit (I get it all the time, and everything still works OK); it looks like it's balking about some problem with your command line. Specifically, I think it's complaining about the -loadidx part. -loadidx needs to be provided with a filename (since it's for loading a video index file that was previously saved by -saveidx). If you have no clue what this index stuff means, take that part out of your command.
Also, I am not sure this command will actually encode anything, because it includes the "-frames 0" option, which means to play or encode 0 frames. There's probably no need to have the "artist=" or "subject=:copyright=:comment=" part, since I think those will be blank by default, and I don't think any of that stuff is saved in an mpeg file anyway, in which case you could leave out the entire "-info" part. (I could be wrong on this, though.)
Finally, if you are attempting to make (S)VCD or DVD format mpegs, I would like to plug my encoder program
tovid, designed for just this purpose.