May xx (between 23rd and 30th), 1970
Not sure it can be called a canceled gig, since this report characterizes plans as tentative. Furthermore, it might well have happened and we just don't know about it.
Further Productions, a Northwestern University student group, was hoping to book the GD for an outdoor gig at a weeklong campus festival planned for May 23-30, 1970. We know the GD were in the UK on 5/24 and though I show a Garcia/Wales gig on 5/25/70 at the Matrix, my next reasonable listing for Jerry is June 1 with Howard Wales at the Matrix. I don't know what's known about their itinerary after the Newcastle Performance, but it certainly seems plausible that the GD could have swung back through Chicago on their way home and played Northwestern, for example on the weekend of Friday (29th) - Saturday (30th).
Recollections one way or the other might be the way to confirm or deny that it happened.
"Zappa and Mothers Give Two Concerts Wednesday," Chicago Tribune, May 3, 1970, p. F7.