Not Applying Credit Memo on Deposit

  If i make a deposit for a customer and then i go to Actions to Credit the Deposit. I generate a credit Memo for the deposit from Credit Balances Button , a note appeares that the balance amount transaction has been credited , then i cannot save the changes and cannot complete the credit memo .
An error is displayed " Unexpected Error has Occurred. Please Contact your system administrator "

I enter credit lines but it does not save the transaction and i have to close the form and the credit memo is also unapplied on the Deposit.If i make a deposit for a customer and then i go to Action to Credit the Deposit.