hello, i think the possibility to add exceptions in an iframe is prohibited by design. in order to work around that try to access the embedded iframe directly by right-clicking the error page, then "this frame" > "show only this frame" and hopefully you can add an exception in the next step.
this should only be necessary once if you choose to permanently store the exception.

What does it say under the Technical Details?

You should always be cautious with creating a permanent exception if there is any chance to solve such an issue otherwise. It is possible that the server needs to update an expired certificate or that there is a problem with a missing intermediate certificate that can be resolved by installing the intermediate certificate from another source. Also check for other possible causes like a wrong date/time setting on your computer.

yes it allowed me to enter exception. Closed firefox then restarted to check behaviour. Still not able to access the frame. If I try adding exception for the frame again, I get the message that this site is already excepted.

thanks for your advice.

is it a publicly accessible page where could share a link to?

it is secure page on my company site accessible only from LAN.

does it show an error code under technical details on the "connection untrusted" page?

Error code: sec_error_unknown_issuer

regards

complete description of error:

MyCompanyinternalwebsiteonLAN:8443 uses an invalid security certificate. The certificate is not trusted because no issuer chain was provided. (Error code: sec_error_unknown_issuer)

thanks philpp, it is working now. I right clicked and selected the frame to "Open AS New TAB" and then added Exception again, which seems to have fixed it. Many thanks for your help. Fantastic Support from you.

Who is the issuer of the certificates?

Issued for internal use by my Company

Can't you contact the IT department and ask them if they can provide a root certificate?