Frequent crash at anim bp compilation

Hi guys,

I am trying to split one big anim blueprint in several blueprints using sub anim instances.
Almost every time I do a modification, engine crashes on TArray::void RangeCheck(int32 Index) const method at folowing line:

    checkf((Index >= 0) & (Index < ArrayNum),TEXT("Array index out of bounds: %i from an array of size %i"),Index,ArrayNum); // & for one branch

with following callstack:

Any idea how I could workaround this crash?
thanks

Quick precision: Crash occurs at blueprint compilation

The issue seem tightly related to nested anim BP.
An anim BP will compile properly, but as soon as the anim BP is used as a Sub Anim Instance, it will crash.
It might be related to following crash:

as I have as well exposed variable from one blueprint to its sub anim instance.

Another precision that could be of importance: recompiling UE4 in debug seem to make the bug disappear