SSL Issue Since the Let's Encrypt Expiry

I imagine this has already been discussed and I’m just not finding it. Since the Let’s Encrypt expiry issue, many of the sites I run through Cloudflare to get the following error from Firefox:

Secure Connection Failed
An error occurred during a connection to ix-online.com. PR_END_OF_FILE_ERROR

If you click “Try Again” the page loads, but the next link you click gives you the same message. Chrome and other browsers don’t seem to care. What should I do to stop this from happening? I use the Flexible option for the SSL/TLS mode.

Maybe it’s not related to Let’s Encrypt?

Then you have a generl security issue.