I created a self signed SSL certificate for the Supermicro BMC which OPNsense is running on. I used the already set up certificate authority (CA) and intermediate certificate of the OPNsense to create the new certificate.
After rebooting the BMC I wondered that there was still a warning for an invalid certificate, but this time I couldn't actually click on "Advanced" and just go on to the logon.
The full error message can be seen in this thread: https://forum.opnsense.org/index.php?topic=13929.msg64113
A guy that probably had the same mistake but chose to load a backup configuration: https://forum.opnsense.org/index.php?topic=8730.msg38862
After a little research and comparing certificates I found the mistake, I accidentally created the certificate as "Client" instead of "Server".
I hope this will be helpful to somebody and save a lot of time. By the way there is a kind of secret phrase in Chrome that you can type on the error page which lets you continue to the login, just type this on your keyboard:
thisisunsafe
This will create an exception for the current site.