Could not establish trust relationship for the SSL/TLS secure channel

Question

I have developed an application which is looking at a 3rd party HTTPS endpoint. On my personal PC (Windows 7) and the customers PC's (Windows XP) everything is working fine.

On another test PC (Windows XP SP3-We are working with some legacy stuff) I am getting the following error.

The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.

This application does not use any client side certificates. There is no code which deals with certificates and I have not manually installed any certificates on any of the PC's.

When I browse to the endpoint on my Windows 7 machine it comes up with a login screen as expected. When I browse to the endpoint on my WindowsXP machine it comes up with the message:

There is a problem with this website's security certificate. 
Click here to close this webpage 
Continue to this website (not recommended).

After clicking Continue to this website (not recommended). the login screen loads as expected but has a red address bar with Certificate Error displayed.

All the solutions I have found talk about installing certificates or changing my code to accept any certificates.

So why is this single test PC failing when the others work without any changes?

Answer 1

I suspect that the 3rd party you are connecting to might have been added to the trusted parties in a windows update, so I suggest - if you are in a position to do that - to run Windows Update on the test computer, also be sure not to skip any rejected or hidden updates. Complete the update process for all Critical and Recommended updates, specifically check and update Windows Root Certificates on your system.

Hope this helps.