I came across this error several times while attempting to put cocked beading on my faceframes (speaking of which, is there a wish list I can add that function to?). The error message in the Constraint Manager bluntly said "Out of Memory", although I have had a more talkative error which mumbled something about Hoops, but I forget which detail I was working on at the time.
As for the root of the midrail/midstile error, I'd make a geometric cut and/or profile, then take the part to the constraint manager to set the dimensions, and it would pop up with the error. Deleting the cut didn't help, however deleting and recreating the edited midrail/stile resolved the issue. The problem was the same, regardless of whether you cut or profiled the piece, and causes problems regardless of whether or not you take something to to the CM. I was able to recreate the error using just the standard faceframe cabinets.
Saving after the error produces an inoperable cabinet; the graphic doesn't move when you zoom (although the target does), and you need to restart the program to remove the graphic from the screen. Saving after editing the midrail/stile, but before opening CM produced the same problem.
I've only been using eCab for 2 weeks, and while I find it's quite powerful and fairly easy to use (after watching and working along with the instructional videos), the number of errors I've gotten, combined with the slow-as-molasses single threading program issue, has been quite disheartening... especially at 4 in the morning >.>