Safari has saved that the Website has HSTS. Now the problem is that the https site was disabled and just the normal http version is used.
HSTS enforced to go to the website via https and it failed.
How to clear/flush/remove/empty the HSTS cache with Safari:
Open the terminal and use those commands:
killall nsurlstoraged rm -f ~/Library/Cookies/HSTS.plist launchctl start /System/Library/LaunchAgents/com.apple.nsurlstoraged.plist
With just the remove command it will not work. You have to stop and restart the service.