Some users are reporting blank screens, 501, or 505 errors upon trying to load Quantity Manager. This is an issue with eBay and can be resolved by repeatedly refreshing the page until it works. In our own testing this can take 20 or more retries but it eventually works. This is the only solution recommended to us by eBay.
For the official announcement from eBay see: https://go.developer.ebay.com/access-issue-open-ebay-applications