Sectigo RSA Domain Validation Secure Server CA removed?

No it's not, that's the reason why I wanted to understand how it came in (i.e. manual upload?) and how it wiped off (need to check) 

I uploaded Sectigo RSA Validation Secure Server CA by hand to get the issued enduser-certs working.
Without I get a cert-error that it's not trusted "Blocked due to invalid TLS certificate".

I just wonder because a lot of websites using certs issued by RSA Validation Secure Server CA and until today I got no errors. It was also not necessary to upload it before.

So I assumed that it was on the firewall before…

Could this certificate be issued by the old CA? 

Can you link us the link of the URL, you saw the problem? 

https://lforr.elainelfo.net/nr243/go/xc0a8rx8k7ekfjejhvi86rg6di2zkohldz74s0s0s1cl/7?t_id=4265649570

(web-content of an paket tracing mail)

https:// lforr . elainelfo . net/nr243/go/xc0a8rx8k7ekfjejhvi86rg6di2zkohldz74s0s0s1cl/7?t_id=4265649570

(the filter here really sucks!)

Something is odd with this website in general.

See: https://www.ssllabs.com/ssltest/analyze.html?d=lforr.elainelfo.net&s=78.35.37.39 

Seems like the hoster of this website made a mistake and used not "the full chain" of the CA. 

Or see: https://www.sslshopper.com/ssl-checker.html#hostname=lforr.elainelfo.net 

firewall has a problem without full chains?

Essentially most client will have a problem with such a Webservice. It is bad practice to build a website like that. You should always include the full chain PEM to the certificate and not forgetting one certificate. 
sslshopper gives you the workaround, you could do on the firewall - But essentially this is a problem of the website owner and his responsibility to fix. 

You can also check here: https://medium.com/@superseb/get-your-certificate-chain-right-4b117a9c0fce