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Support responded with the following:
Please perform the following steps on the second computer:
1. Try disabling your antivirus and firewall.
2. Open/forward the following ports:
Uplay Launcher:
TCP: 80, 443, 13000, 13005, 13200-13999
3. Disable startup programs:
Start -> Run (or press the Windows+R key combination) -> In the window that appears, type MSCONFIG and press Enter -> Go to the Startup tab -> Select the "Disable All" option -> Click "Apply", then "Close", then restart the system.
(Keep in mind that this is only a temporary solution and any changes can be undone by returning to the MSCONFIG utility and selecting "Normal Startup")
4. Go to the folder C:\WINDOWS\System32\Drivers\etc. Right-click on the file named hosts and delete it.
5. Make sure you are not connected to a proxy server.
6. Emulation programs (Cyberlink, Alcohol 120%, Daemon Tools, Nero, etc.) can also cause conflicts. These programs must be removed or disabled while launching the game.
Referring to the post above, I have already done this – it doesn't help.
And most importantly, the problem is recent; everything was fine before.
I am waiting for the next response from support.