I'd submit a Support Case to Auctiva, so they can look at the API error. I'd include a link to one of your active auctions on ebay, so they can see you are having success with direct submissions to ebay. Hopefully, they can figure-out what word or restriction is causing the API.
It could be a bug, but my best guess is......ebay is doing more intensive screening of 3rd party submissions to prevent fraud or abuse (not that I'm suggesting you're doing that ).
Whatever the cause, please post back what Auctiva finds as the cause. ChooChooGuy might be correct on the word "animal", but I have a gut feeling it's something else.
Danno