For the researched here, there is no way to use this certificate by axigen.
Cloudflare does not allow proxy for MX record but requires proxy for SSL, as it has to go through his servers to be trusted.
So in this case I would have to use another certificate.
I tried to do the creation via LetsEncrypt, but it stays on this screen loading forever.
Logs:
2020-08-22 03:17:09 +0200 08 vps159681 WEBADMIN:00009C66: Let’s Encrypt: Issuance Job added successfully
2020-08-22 03:17:09 +0200 08 vps159681 WEBADMIN:00009C66: LetsE: Found current request
2020-08-22 03:17:10 +0200 08 vps159681 JOBLOG:70000031: LetsE: Acme job executing
2020-08-22 03:17:10 +0200 08 vps159681 JOBLOG:70000031: LetsE: Found current request
2020-08-22 03:17:10 +0200 08 vps159681 JOBLOG:70000031: LetsE: AcmeInitState for ninjascalper.net executing
2020-08-22 03:17:10 +0200 08 vps159681 WEBADMIN:00009C66: LetsE: Found current request
2020-08-22 03:17:10 +0200 02 vps159681 JOBLOG:70000031: HTTP-Client: Error performing request in connection to https://acme-v02.api.letsencrypt.org:443/directory:SSL peer certificate or SSH remote key was not OK
2020-08-22 03:17:10 +0200 02 vps159681 JOBLOG:70000031: LetsE: connection error on GET when populating acme link directory
2020-08-22 03:17:10 +0200 02 vps159681 JOBLOG:70000031: LetsE: Job step action => Connection-related error, re-attempting after 15 seconds