Causes
The profit from Balance sheet and Profit & loss statement (balance from Previous year column) don't match. In the Accounts table, in the Previous view (and Previous column), balances have been manually entered with some mistakes.
Solution
Check the accounting file with the Check accounting command from the Actions menu. In the Accounts table, in the Previous view (and Previous column), check that all the balances are correct: the costs must be entered in positive and the income must be entered in negative (with a - sign).