Payments in Classified not working properly

Hi,
I have encountered some problems with the payment of ads. I have recurring payments enabled and the Billing period is set to week and billig frequency is 8, that makes 8 weeks for each billig cycle doesn't it?
Each ad has a duration field that has 8 Weeks as its only avaiable choice. When adding a new ad from frontend with another user account with subscriber role, I get this red error message:
"You do not have enough credits to publish your classified for the selected time period. Please select a shorter period, if available, or purchase more credits.
Your ad has been saved as a Draft."

I don't understand. I am not useing the credit system at all and the billig selected time period is the same as the billing period.
What is the problem?