SSL errors accessing specific website

My work uses Constant Contact, and I visit it often, but recently I have had problems. There are Landing Page automations that, when someone clicks the URL on my website, they are directed to a registration page on Constant Contact, then are given the download.  The URLs for the landing pages are in various places on my website.

Recently, whenever I click on one of the landing page links, I get a browser error:

This site can’t provide a secure connectionlp.constantcontactpages.com sent an invalid response.

Try running Windows Network Diagnostics.

ERR_SSL_PROTOCOL_ERROR

It doesn’t matter if I click the link on my website or within the setup on Constant Contact’s website; I get the same error.

After some troubleshooting, it turns out that my computers are the only ones getting this error – people not on my home network have no error; even my cellphone gets the error if it is on Wi-Fi, if I remove it from Wi-Fi, it works with no error. It seems obvious that something in my network is causing the error.

I’ve tried several things:

• Rebooting the router
• Rebooting my switch (PCs are on ethernet)
• On 2 different PCs
  • I tried Chrome and Firefox
  • Clearing browser cache
  • Disabling all extensions
  • Incognito mode
  • Deleting every Constant Contact-related cookie off the computer
  • Verify system clock is accurate
  • Clear SSL state
  • Disabling Anti-virus and firewall (Norton 360)
  • Verify Windows up to date
  • Verified browsers up to date

Any ideas?