The mod Rampant, fixed (2.2.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event RampantFixed::on_unit_group_finished_gathering (ID 157)
value for required parameter 1 (member) is missing
stack traceback:
[C]: in function 'add_member'
RampantFixed/libs/SquadCompression.lua:402: in function 'squadSmoothCompress'
RampantFixed/libs/SquadCompression.lua:452: in function 'processCompression'
RampantFixed/control.lua:947: in function <RampantFixed/control.lua:923>

This is happening randomly but frequently. I'm using a lot of other mods as well so it could be a conflict.

It seems the game failed to spawn a biter at the specified location when attempting to "unpack" the unit.
I've done an additional check, but the release is not ready yet. Most likely it will be released on September 4-6