I believe some sandbags either burned up, or were destroyed in a repair zone, or deconstructed and they job to repair them stayed, locking up random colonists. I couldn't see a job for it in the new job list either. To unlock the colonists, I had to recruit and release them; but they would eventually get locked up again.
To eventually resolve the problem, I built sandbags everywhere I thought the job might be pointing to and then deconstructed them. That seemed to fix it.
I believe some sandbags either burned up, or were destroyed in a repair zone, or deconstructed and they job to repair them stayed, locking up random colonists. I couldn't see a job for it in the new job list either. To unlock the colonists, I had to recruit and release them; but they would eventually get locked up again.
To eventually resolve the problem, I built sandbags everywhere I thought the job might be pointing to and then deconstructed them. That seemed to fix it.