Use the attached test mission.
The spawnAI module is set to only spawn 1 infantry "max Soldiers per group" and its manpower cap to 0.2 ( A blufor B_soldier_F's manpower rating is 0.4, ) so nothing can be spawned causing the error.
Change the manpower cap to > 0.4 and everything will work ok.
This is set up to specifically show the error but this can happen at any time especially when using armoured groups where their manpower rating is a lot larger ( e.g B_MBT_01_cannon_F is greater than 23 for the tank and its spawned crew ).