Klarna Account and Invoice

I have built an online shop using Joomla 3 and Mijoshop (Opencart version 1.5.51). Everything has worked out fine two years, but now the clients can't use Klarna payment anymore (Error code 9114). If I try to change the live/beta mode, I get an error and the log file says "Could not retrieve pClass for FIN. Error Code: 9113; Error Message: An integration problem has occured between the Store and Klarna. Please contact the Store for more information or choose another payment method."

Klarna can't find the problem, the Merchant Id and secret are ok. Nothing has been changed in the shop or my server. Any ideas?