Hi Bart,
Based on your description this error sounds account specific and we in Support would likely need to see your account in order to determine the issue you are having. Could please send me an email, either through these forums or the email address in my signature, include your contact information, Company Account name, and if not all items are affected, please list a couple items with this issue. I would also need you to make sure Method Support is turned on for your account, this can be done by going to the Customize-Users screen, locate the MethodSupport user and make sure there is a checkmark in the active column for this user.
As soon as we get this email from you, we will be able to troubleshoot this issue and get in contact with you with our findings.
-Ben