Ubisoft is currently in the process of moving their online services in-house, off of third-party servers, and while the company previously laid out which games would remain online (and which would not), they've hit a few snags since.
Unexpected issues have hit the PC version of Driver: San Francisco, as reported by players on Ubi's official forums, though the game does have an offline mode. Some have also noted problems with Ubisoft's futuristic strategy title Anno 2070 and Assassin's Creed Revelations, though all aforementioned games were said to be "safe" during maintenance.
Such inconveniences are certainly expected during the process, but Ubisoft has issued an apology via Twitter to their player base regardless.
"We apologise for the inconvenience, it seems some of you can't connect to games announced as playable during migration. We're on it and will keep you posted here!"
PC games expected to go completely offline during the shift include Might & Magic: Heroes 6, The Settlers 7, Tom Clancy's HAWX 2, and Mac versions of Assassin's Creed, Splinter Cell Conviction, and The Settlers. Ubi's Uplay service is also unavailable.