The URL is as it seems...the URL of the dispatcher agent's invalidate cache operation. For example (using SSL port since mine happens to be set up this way at the moment):
What part of the URL are you changing? What does the log say? What happens when you test the connection?
Justin Edelson providec the answer to me (thanks Justin!):
If you use path-based virtualhost entries to differentiate between farms, you use this field to target the farm to invalidate.
I.e. If you have farm #1 with a virtual host of www.mysite.com/path1/* and farm #2 with a virtual host of www.mysite.com/path2/*, you can use a URL of /path1/invalidate.cache to target the first farm and /path2/invalidate.cache to target the second farm.