I love support calls where I learn something new.
I've never seen a 509 server response before. 500 errors mean that the server was unable to fulfil a valid request, and the 509 means "Bandwidth Limit Exceeded"(not used by all servers).
In this case it meant that Integrity was hitting a site too fast for the server's comfort. Easily solved here by moving the 'Threads' slider over to the left, but if that hadn't fixed it, it's possible to further throttle the crawler by specifying a small delay between requests: