That error appears when the operation initiated via the debugger (in this case, Dump) takes too long to execute and the debugger is unable to abort the operation. During the wait, LINQPad's debugger remains responsive due to it being asynchronous, but it still has to report the error, because the failed abort prevents the query process from continuing normally and requires a process recycle.
After trying your sample, I can successfully dump the WebException, which means I can't identity the cause without extra info. Can you try a couple of things:
1. Run the same thing in VS. Does the debugger freeze for a while, or give an error?
2. Click the minus sign on above the Dump. If you change the dump depth from 3 to 1, does it work? And if so, the output should contain hyperlinks that you can click. Does clicking any particular hyperlink cause an error? Sometimes particular property accessors are problematic and I can tell LINQPad to avoid calling them.