Cause
There are errors in the Accounting (possible differences in the accounting, missing BClass for an account, or maybe missing GR reference for an account)
Solution
Running Check accounting (enable all options), will produce an Info report (Information window) visualizing all possible errors. Double-clicking on the error message will direct you to the row where the error occurs.
Correct the error and rerun Check accounting (Actions menu), until there is no more error message.