Hi Andrea, sorry for the delay in replying to your question.

Does the error page have an "Advanced" button? If you click that, Firefox should show an error code similar to SEC_ERROR_BLAH_BLAH which will help in diagnosing the problem. There's more information in the following article:

How to troubleshoot security error codes on secure websites

Are you possibly using a VPN or proxy ?

You can remove all data stored in Firefox for a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data for that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data for that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

I reinstalled Firefox from scratch and everything works fine now.