I can't speak to this particular "insane error", but most that I have run into in 5.1 were caused by LabVIEW getting "lost" due to some odd data corruption in the vi I was working in. If the vi that is giving you trouble isn't too big, one, annoying, method to get past this is to start a new vi and duplicate the old one. Don't copy and paste though, it will sometime just copy the corrupted code. Save it before you close the old one (so that you don't crash LabVIEW)
Good Luck, there is a reason that they call them insane errors, they drive you insane! LV 2.5.2 would get them about every hour, or just before you saved your code, if you were doing intense coding, particularly as the diagram got a little complicated. Helped teach me to make sub-vi's!
Putnam Monroe
PutnamCertified LabVIEW Developer
Senior Test Engineer North Shore Technology, Inc.
Currently using LV 2012-LabVIEW 2018, RT8.5

LabVIEW Champion