...and a final item to add to Steve's list...

You are running an older version of Adagio Ledger (check your service packs), and the batch you are trying to unpost orignally posted with some transactions being thrown into an error batch. In this case, the amount recorded as posted in the batch control file does not agree with the amount of transactions that are found and the error you see is generated.

The only way to fix this is through a data repair or by judicious use of Adagio GridView-RW.