I remember I tapped ctrl-z 3~4 times “very quicky” and it takes quite a long time for ue4 to process it before the bug appear. And the bug remains even if I re-open the editor.
After that, I just avoid using ctrl-z as best as I can.
The bugged blueprint is my character blueprint, and before the bug appear, I was editing my character’s IK related custom functions, don’t know if this help.