That probably means that you have an older version of PRTG which still uses the deprecated and broken SSLv3 protocol, which has been deactivated in recent versions of major browsers. I don't know if there's a way to turn it back on (because it's a terrible idea), you might need to either use an older version of Firefox that still supports SSLv3 or disable encryption for the PRTG web server (as that is no less secure than using an encrypted connection with SSLv3) to access the web interface again.
Once you gain access, I would STRONGLY advice to update your PRTG to a version that disables SSLv3 (14.4.12 or so, I think).
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