It's 100% tested thing and happens every time, looks like a bug.
So let's suppose I have 2 or more vMix Call inputs active. If I change output for call_1 (Out_1 > Out_2), and at the same time Out_2 was already used by another call_2, then picture on call_1 crashes.
When I change Output for call_1 back on Out_1 or any other Output, that is not used, then picture is fine. It doesn't matter how many vMix Call inputs active or which Output is switched for them. The only thing that's matter is whether Output is already used by another vMix Call input.
I've found the only cure for this - reconnect call and do not change output to prevent problems if several calls using the same output at the same time.

It is quite a major problem and I hope it will be fixed.