Probably you have already a row with intStatus 18 and uid 'a1a1...'
Try to run and see if you have already the row that you try to create with your UPDATE:
SELECT uid,
intStatus
FROM dbo.MYTABLE
WHERE intStatus = 18
AND uid = 'a1a1a1a1-a1a1-a1a1-a1a1-a1a1a1a1a1a1'