LOTW fail to upload

[VK2MET]

After >1,000 QSOs uploaded to LOTW, I suddenly get this error:
This log has invalid QSO information
The log being signed has 'IOTA' set to value 'OC-001' which is not valid on line 11

I've checked previous QSOs and they all have OC-001

What the devil? Where do I look?

Alan

[VK2MET]

Deleted all the listed awards and it uploads - no idea why this one does not go