Hello! What are your return policy settings right now? I just posted a listing with a return policy and it went through ok--ever since eBay decided to make every listing specify a return policy, auctiva has to make changes to try and keep up with it.
I chose "Returns Accepted" and item must be returned within 7 days; refund will be given as Money Back ; and the return shipping will be paid by the Buyer.
If you've double-checked your settings and are still getting the api error message, definitely file a support ticket...good luck!