Site not loading after upgrade Lightsail instance

[fritex]

Regarding Cloudflare 521 error, may I suggest you to take a look into below article to troubleshoot the issue, therefore if you already did may I ask what have tried so far from the suggestions below?:

Community Tip - Fixing Error 521: Web server is down

Error Try the suggestions in this Community Tip to help you fix Error 521: Web server is down. Background A 521 error happens when we are unable to make a TCP connection to your origin server. Specifically, Cloudflare tried to connect to your origin server on port 80 or 443, but received a connection refused error. This is often caused by security or firewall software and happens if the origin server has directly refused Cloudflare’s proxy request. [Banner in tip catalog] Quick Fix Ideas …

@nawaz

after upgrade

Maybe something at the Lightsail firewall changed or need to be fixed?
For example, kindly re-check if Cloudflare is allowed to connect to your origin host to as follows in the below article:

• https://support.cloudflare.com/hc/en-us/articles/201897700-Allowing-Cloudflare-IP-addresses

Nevertheless, Cloudflare IP addresses list can be found here:

• IP Ranges

Before moving to Cloudflare, was your Website working over HTTPS connection?