I am sorry you get an error when trying to access Connect through Garmin Express. The error you get is not common so I would like to get some log files from Garmin Express. Please do the following:
Open Garmin Express.
Click Help in the menu bar (gray bar at the top of the screen).
Click Garmin Express Help.
Click Send Reports.
Enter the code K6M4V.
Click Submit and let us know you sent them.
When we can see the logs, we can better determine the cause of the error.
Thanks, just looked through the FAQs (didn't see them before) - Auto Proxy Discovery was disabled, i've enabled it but it still won't see the servers. David, I'm unable to resend logs after enabling Auto Proxy Discovery.
Thank you for sending the requested logs. The error you are getting in the logs (403 error) can be because the device is already saved in Connect. Please follow these steps to remove it from Connect:
Log into Connect.
Click on the Settings menu on the left (the 3 bars).
Select "Devices".
Select your Forerunner 310XT.
To the right of the Forerunner 310XT at the right of the window, there is a gear icon. Please click on it.
Select "Remove" and confirm you want it removed.
Now, please try adding the device to Garmin Express and Connect again.