UPDATE 27/01 16:55 GMT: Problem solved, apparently.
"We looked into this with BT Infinity and were able to solve the problem," Ubisoft told me. "BT Infinity customers should now be able to connect to Ubisoft's games and services as usual."
ORIGINAL STORY 24/01 14:25 GMT: For some reason, BT Infinity customers in the UK are having a real bad time of connecting to the Uplay servers. In other words, they can't.
Readers wrote to tell us about the issue, and highlighted a Ubisoft forum where complaints are piling up. And no one seems to be able to get a straight answer.
There's hope, though, as Ubisoft told me this afternoon it was aware of, and investigating, the issue. BT Infinity I was unable to get hold of, but hope to hear something soon.
Uplay is the service Ubisoft uses to handle online modes in its games. But Uplay can also crop up in single-player, such as in Assassin's Creed 4 in Kenway's Fleet resource-gathering mode.