Hmmm the error re-occurred this morning on the same record. I've re-run idxfix and it reported that nothing needed fixing this time. And yet, when I try to delete the record I still get the error.
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Error
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SYSTEM ERROR: Index ErrorCostingIdx in Costing Details for recid 1141752647 could not be deleted. (1422)
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OK
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Any ideas? Ideally I don't want to have to take the db offline.