Content Filter ist not warning on unrated webpages: Bad Request

We got a firewall USG Flex 700 with a contant filter active.

If we try to oben an unrated website we get following massage:

Bad Request
Your browser sent a request that this server could not understand.
Reason: You're speaking plain HTTP to an SSL-enabled server port.
Instead use the HTTPS scheme to access this URL, please.

(we set unrated sites on warn, not block)

Normaly it should show an info, the site is unsave and not block it completely

Anyone know how we can fix this? We want to be warned.