I don't have a server with Apache and HTTP/2, I use only NGINX with HTTP/2. By itself a cert hardly can cause an issue with availability of the site (if the cert is trusted of course). What might matter is a list of CERTs and OpenSSL version.
I am using nginx as reverse proxy, however now my Openssl veriosn in 1.0.2k which is latest one as per website and is enabled as default, problem is resolved by disabling http2 in both nginx and apache. i tried comodo version on one of website i can send you link , i got A+ for this ssl certificate also.
Kindly tell me any further information which can be needed.