I've had a few users report receiving the following error when logging in and getting the authorization token for the cafeToolbox PF Feed Generator tool to access their account.

-- BEGIN ERROR MESSAGE --
The API key or signature is invalid. Please contact the application developer for further assistance.
-- END ERROR MESSAGE --

I can reproduce the error if I am logged out at printfection.com and initiate the cafeToolbox PF Feed Generator AND using Internet Explorer 6.0. I've tried it on two different computers with the same IE version and different internet connections.

When I first click the link, I am prompted to log in at printfection.com.
After logging in, I receive the error. If I go back to cafeToolbox and re-initiate the feed generator request, the error does not appear (I presume because the user is already authorized and the Printfection API sends back the auth_token so the cafeToolbox script can continue.

Performing the same steps in Firefox don't result in an error -- and the entire process works as expected.

I'm not sure if this is a bug, but because it works without problem for me in Firefox. Does anyone have insight on what the problem might be?

I'm beginning to believe this is a problem where the ISP is running users through a proxy. Any way I can deal with this from a site/scripting basis?