Had this issue myself. It seems to be a known issue, root cause is still being investigated.
2017.R4.P6
In my incident, error is generated due to service level data, (expected call volume) in the forecast not being passed to the schedule. I also see that non-ACD activities are stacked. All agents are assigned a break or meal at the same time.
Remediation:
Take just
one agent in each
scheduling unit, and remove the
Optimizer schedulable license form that one agent for 15 minutes, then re-assign, wait 15 minutes.
Regenerate the schedule.
I suspect a standby server re-boot, manual switch over will also remedy this, albeit not as gracefully.
My suspicion is that when agents are removed from IA while still assigned to an optimizer scheduling group, yet this is just a guess. I am used to older phone/ACD systems where multiple databases must be managed, and an entity must be removed object by object, rather than simply deleting the agent.