Looking at the code, the problem is pretty obvious. A recent patch

Looking at the code, the problem is pretty obvious.  A recent patch
to init.c (the one that was supposed to prevent Mutt from silently
failing to read nonexistant RC files, IIRC) changed source_rc() to
stat(2) the RC "file" before trying to mutt_open_read() it.  There's
only one problem here: source_rc() has no way of knowing whether or
not its "file" is a file or a command.  I'm attaching a patch that
fixes the problem in what I believe is the right way.