Ue crashing when "apply" in animation

So im new to game dev, and i ran into this realy weird bug. Everytime I change something in an animation and hit apply afterwords, the engine says “saving packages - 0%” wait for a couple of secs and than it crashes. The error message I get in the bug reporter says: Assertion failed: (Index >= 0) & (Index < ArrayNum) [File:D:\Build++UE4\Sync\Engine\Source\Runtime\Core\Public\Containers/Array.h] [Line: 614] Array index out of bounds: 17 from an array of size 17

What can I do about this?

and when I wan´t to dublicate an animation, basicly the same thing happens.

Edit: After restarting Ue the problem sometimes goes away