CiviCRM for Outlook - Invalid Credentials
I'm not a civi expert by any means, but thought I would share my experience - I had a similar issue, and it turned out to be a firewall issue - civi was hosted on a machine on the net, and my Outlook client was on our corporate network. When I was connected to our company network, I would get that error. When I took the machine home and tried connecting there - it worked fine.

Other things that got me to start with - in the URL path, make sure you have the trailing / on the path   (check by copying the url and trying to access the url in a browser), and make sure you're using the correct site key from the civicrm.settings.php file

Hope something here helps!