I was using Mob Rebirth. However, when I killed a Mutant Zombie, the game crashed.
I wonder if this can be fixed, Sorry that I cannot provide the crash report because this crash happened long ago (and I was using old version at that time.)

uhh, the exact time may be January or February this year.

Which version of Mob Rebirth? I tried it with the latest 1.12.2 version of Mob Rebirth, and I did not get a crash.