Firefox is unable to load a secure website and refuses to either continue with manual approval or fails to add exception while it confirms the certificate as valid.

When clicking the link below, firefox will move to the site but prompts the user with a notification that it cannot verify the secure connection. Technical details state the certificate is invalid (no other useful information there) and the button "get me out of here" reveals a screen where there is a message stating the certificate is valid and there is no need to add an exception. All buttons are greyed out then, and thus preventing to access the site. No overrule or any other method works, so we (= our company) are forced to use an other browser to access our suppliers e-learning environment. I've tried to modify the two flags in about:config, but apparently those are depricated since they don't affect behaviour. (browser.ssl.override_behavior and browser.xul.errorpages.expert_bad_cert) - edit: apparently the link falls off. this is the link: https://wbt.progress.com/