Just thought I'd contribute a fix that actually worked for me. I'm running Win 8.1 64-bit. Garmin Express downloaded and installed, but had the "your internet connection is being blocked" popup. I followed the sticky instructions to change the logon credentials for the Garmin Core Update Service.
HOWEVER - instead of just removing the password that was pre-filled, I had to enter my actual computer user account password.
Garmin Express has now worked for me - successfully uploaded data from my Forerunner 15 and had two updates pushed to the device. Data pushed to Garmin Connect as well.
