When I start my uplay client to play some ACII and try to login with valid credentials (tested on the uplay website) I get "A Ubisoft service is unavailable at the moment.".
I've googled the message and tried every solution I could find. Even created a ticket over at uplay's support website (which is currently malfunctioning). And got a very generic response saying it's probably my firewall, my host file or my router. It goes as far as suggesting I should open up ports 80, 443, etc.
In an attempt to save someone some serious head scratching, I will share the solution (that worked for me) below.
Best Answer
Turns out the message "A Ubisoft service is unavailable at the moment." is way off. In my case, it had absolutely nothing to do with a service being unavailable, connectivity was perfectly fine. The uplay client is just not getting the response it's expecting.
The reason I claim this is because I remembered I had a second account with which, to my surprise, I was able to login to the uplay client just fine. Let me refer to my accounts as "Account A" with which I couldn't login and "Account B" which magically let me login into the uplay client.
When I logged in with "Account A" on the uplay website there was an error stating "countryCode is not ISO-3166-1". I initially ignored this since the whole website is buggy and clicking "Ask me later" got me to my account page. But when I noticed I didn't get the message when logging in with "Account B", I got suspicious. For "Account A" I made the mistake of selecting the wrong country when I signed up with it, "Netherlands Antilles" when I actually live in "The Netherlands".
After contacting support, asking them to change my country (something you can't do through your account settings ...) to "The Netherlands" I was finally able to login with both Account A and B into the uplay client.
If you're wondering why I really wanted my "Account A" to work, it's because I was able to activate and play ACII with "Account B" exactly once. When I booted the next day, to play some more ACII with "Account B", I received a pleasant "This CD Key or Activation Code is already in use with another Uplay Account". Turned out the game was, somehow, activated for "Account A". So I really wanted that account to work.