I'm getting
errors: [ { errorId: 30500, domain: 'API_FULFILLMENT', category: 'APPLICATION', message: 'System error.' } ]
returned on the getOrders call, in an app that previously worked just fine.
Interestingly enough, this app connects to three ebay stores, and one of the three actually appears to be working fine, though there doesn't appear to be any authentication issues in place here.