Ahaaaaaaa..... ok, thank you very much, gentlemen, for pointing me in the right direction - everything's working again!
Adam B.'s suggestion No.2 didn't work, don't know why; but installing mrxvt (btw, how do you pronounce that??? Like the similarly named Batman villain whose exact name escapes me at the moment?) ultimately led me to, ahem, the
amazing discovery that I shouldn't have stuff sitting in sub-folders ... see, when shell access became available the easy way just recently, I downloaded everything I needed and installed it - and in some cases changed program locations afterwards. Seems that neither gnuchess nor emelFM2 will run when dropped into something called /notes/shell access. I think I read in connection with one of the community-developed programs that "it doesn't matter where you install it" and unconsciously applied this to all of them. I'm sorry I voiced the suspicion that installing jukebox had anything to do with the problem when it was just fussy ol' me rearrangeing my files and folders...
Again - thanks, kudos, assorted grateful bows - couldn't have figured this out by myself because nope, I know next to nothing about Linux ... it is faintly reminiscent of DOS, is it not? What with the
cd.. and all that....