TLDR:
- Disable your network/unplug your Ethernet port
- Launch the game from Steam/UPlay
- Press skip
- Enable your network/plug your Ethernet port
- You can play online/offline now
I've encountered this as I've seen countless others do. The generic lazy response is "Well you launched the game with another UPlay account, your fault." I'm here to tell you that this isn't the case. Even if it is so, that's not good customer service.
I encountered this error with Far Cry 4. I played through 3 recently (I know, late) and I was pretty happy with the game. I wanted to try out 4 now but was met with this error. I went through support and was asked if I had any different accounts. I provided them with all the possible IDs, emails, and even a CD key for TrackMania 2 Canyon that I played at some point. They couldn't find that key, which is odd. I installed that game and found my username and email, which is the same email I am using for the current UPlay account. TrackMania was using a different system though, not UPlay. Since I played Far Cry 3 before Far Cry 4, I couldn't have possibly attached it to another account and Far Cry 3 worked flawlessly. The game was not attached in my UPlay account, but it was in my Steam account. They added the game manually and it still would not work.
I spent about 4 days working with them. Doing firewall changes, port forwards, configuring all the .exe files for both UPlay and Far Cry 4 to run as admin by default, uninstalling then installing from each of UPlay and Steam, re-installing both apps, flushing the configs for Steam, and deleting files from UPlay and re-launching. None of this helped. Not a single one.
Throughout my own digging I found out that if I start UPlay in offline mode, it'll start the game no problem. I had no idea why that is, because it technically should fail since the authentication service wasn't called. I guess it was a design decision to let players enjoy their games in offline mode. So I figured let me try something: I turned off my Ethernet port (Disable network or unplug) and launched the game. It'll run UPlay and try to either sync your game saves or your achievements with no luck. It'll eventually ask you to either skip or try again. Now it is your choice, you can plug/enable the internet back again and press try again and it'll load and start the game, or plug/enable the internet and press skip. In either case scenarios, it'll launch the game and you'll be able to play online. Your UPlay account will show up, you can start a campaign mode online and do whatever you need.
IMO: Ubisoft doesn't know how to build a robust gaming service. This authenticator clearly has issues, whatever they may be, and it's absolutely awful that customers have to face these types of issues and then get blamed for it.
Do with this information as you wish. Good luck.