Update 2/19/21: IL has updated their system and can now process rejected IL returns. You may resubmit your rejected IL returns at this time.
Update 2/17/2021: We are seeing this happen for both TY2020 and TY2019. We have reached out to IL for guidance
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Illinois returns are currently getting rejected for ILHEADER-2400-5:
All IL returns will get rejected until we release a fix. Once the fix is released. The rejected IL returns will be listed in the Rejected Returns list so they will be easily accessible when the issue has been resolved.