Found a thread on the Ebay Developers Program and it sounds like the error pops up when 5,000 call/day through the API have been used. I don't know, but I'm guessing that since all the AW2K users use the API, that each listing across all users counts as 1 call, maybe? Then again, it could just be something on Ebays end. Anyways, here's the post I found: https://forums.developer.ebay.com/questions/540/serious-error-518-call-usage-limit-has-been-reache.html