Firefox 58 Windows does write

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Secure Connection Failed

An error occurred during a connection to foreman.ontram.de. You have received an invalid certificate. Please contact the server administrator or email correspondent and give them the following information: Your certificate contains the same serial number as another certificate issued by the certificate authority. Please get a new certificate containing a unique serial number. Error code: SEC_ERROR_REUSED_ISSUER_AND_SERIAL

The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
    Please contact the website owners to inform them of this problem.
---

after accessing https://foreman.someurl.de.

It's not possible to go to advanced and accept the SSL certificate.

Probably it's something with the SSL end date?

"The certificate will not be valid until (date)"

https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean