I've found several instances of people having similar problems to the one that two of my users are having, but I haven't seen any solutions to these problems, yet.

My users are able to click the log-in to Salesforce and enter a username & password, but they get the error dialog:  "Unable to send request to server. The system cannot find the file specified."

They are using the security token.  As a matter of fact, the same happens no matter what you type in the username/password fields, so I don't think any connection is actually being made to saleforce to do validation.

I've totally uninstalled the add-in and reinstalled it again, but it made no difference.

Searching the Salesforce help, all I could find was reference to proxy settings in IE, which seemed odd to me, but I tried it anyhow... no help there, either.

Has anyone actually ever resolved this?

Any suggestions on where I should look?

I tried to edit the add-in code to see what was happening there, but it is password protected.