Hi lynlis,
I'm sorry to hear that you are having trouble with the eBay API error message indicating that you must sign eBay's new international selling agreement before posting additional items with international shipping, but I have to agree that the root cause of the error must lie on eBay's side of things as you mentioned being informed of when you contacted our customer support team.
The API error that you see when you click the red number in the Notifications section from your account is actually the full error message that eBay returned when they rejected this listing and we do not have access to any additional details about why the error was returned.
Based on the fact that you are receiving this error when you try posting items with international shipping through Auctiva and you are able to add those international shipping options through eBay's “Revise your Item” form, it seems that eBay's API data for your account must incorrectly reflect that you have not accepted the new agreement.
I did notice that you generated a new token to renew the association between your eBay account and your Auctiva account since receiving the last error of this nature so perhaps that will update update the apparent incorrect data and prevent you from receiving any more of these errors.
If you continue to encounter these errors with new listings that you attempt to post with international shipping moving forward, I can only recommend that you continue to follow up with eBay's customer support team for further assistance with the matter though.
-Mike