Error 526 - Invalid SSL certificate but it's valid

I also tried to purge cache and no change.

SSLShopper says “certificate is valid”:
https://www.sslshopper.com/ssl-checker.html#hostname=drophost.tk

My virtual machine OS is ubuntu 18.04 with webserver Nginx.
No errors on my VM.

Invalid SSL is coming from nginx origin and not CF edge. Try switching from Cloudflare Full Strict SSL to Full non-strict to see if it resolves. If it does, then your origin SSL cert on Nginx server is the invalid SSL cert