A little bit of grep-fu will lead you to line 549 of minos-base.fs...
Code:
545 \ XResource 17sep07py
546
547 : open ( string -- ) [ also DOS ]
548 6 0" " setlocale 0= abort" Cannot set locale"
549 XOpenDisplay dup dpy ! 0= abort" Can't connect"
550 XSupportsLocale IF
551 s" XMODIFIERS" env$ drop ?dup IF
552 XSetLocaleModifiers 0= IF
553 ." Warning: Cannot set locale modifiers to '"
554 s" XMODIFIERS" env$ type ." '" cr THEN THEN
555 THEN [ toss ] ;
...so there's something off with your X environment in that shell (or the way Minos accesses it), since he's reporting that the call to XOpenDisplay is returning a zero. You can check the man page for the XOpenDisplay call to see what it is doing.
I've never run BigForth and I can't do so on my 64 bit system, so that's all the "help" that I can provide. Sorry.