My bad, it's actually when opening the army window, ctrl+a

Error while running event enemyracemanager::erm-open-army-window (ID 209)
enemyracemanager/gui/army_control_window.lua:654: attempt to index field 'erm_army_control_toggle' (a nil value)
stack traceback:
enemyracemanager/gui/army_control_window.lua:654: in function 'toggle_main_window'
enemyracemanager/controllers/custom-input.lua:69: in function 'handler'
stdlib/stdlib/event/event.lua:342: in function 'pcall'
stdlib/stdlib/event/event.lua:362: in function 'dispatch_event'
stdlib/stdlib/event/event.lua:438: in function <stdlib/stdlib/event/event.lua:396>

The crash should be fixed in ERM 1.16.6. Pressing it will do nothing in the new release.

The army windows only works when "ERM - Terran Units for Players" is active.

https://mods.factorio.com/mod/enemyracemanager