Visual Studio 2008 won't let me change a web reference

Question

I have a web reference in my solution, and I want to change it to point to a new URL (for testing). Some of the time (~75%) when I paste in the new URL in the properties window, I get an immediate popup: "Property value is not valid" and the details say "Exception from HRESULT: 0x80CB7EC8".

The new URL is valid (I'm pasting it). If I reboot or restart VS it will sometimes work, but not always.