After ACE3 3.2.0 update and Arma 3 update 1.48 from Bohemia (described in SPOTREP #00046) on 2015-07-29 on the previous night, a weird issue occured to me. Here is the fix.
Experience:
#0 You update your Arma 3 AhoyWorld repo for modded operations on EU#3 on 2015-07-29, fetching the ACE3 3.2.0 updates. You got a "steamrolled" Arma 3 update 1.48 on the previous evening. After successful and complete download of everything you attempt to get on EU#3 using ArmA3Sync and get "Session lost".
Fix:
#1 Start Arma 3 using the Arma 3 Launcher coming with the game once: start the game using the launcher once and exit the launcher without any further need to start any game with it.
#2 Go back to ArmA3Sync, launch the game as in step #0, this time you get on EU#3 without problems anymore. The first start of Arma 3 using ArmA3Sync like described in step #2 may take a few minutes, might be BattlEye updating in the background.
Thanks to alganthe for figuring out the fix.