I've had this problem before but can't remember the (so-called) solution.
Logged in to BT-FON. I'm on the mobile-only option, so I don't have a BT ID for encrypted nodes (which is another issue entirely).
When I try to go to a variety of websites (cryptocurrency sites seem to annoy it, for example), I get the BT Parental Controls from someone else's setup - mine are not switched on because I don't have any with this subscription (no router, no BT ID).
I've logged out and left it for 15 minutes, flushed DNS cache, logged back in... same thing.
Anyone have a link to the solution or remember it off-hand? Couldn't find it with a search. 😞