I work on a site that uses S2 member Pro for membership with paypal payments.
Some users are experiencing the above error when redirected back to the site from paypal. Although this is a paypal API error paypal support have suggested we contact the plugin author. I thought this community may be able to help.
Has anyone experience this. It is hard to replicate in most instances but one particular user always gets it when testing with their paypal account, and some customers are also getting it.
Thanks in advance and let me know if I can give any more info that may help