[Update - I experienced this part of the game myself, and can safely say that as long as the Templar is your follower officially, you should be ok on this one. I personally waited until my companion was level 13 before testing this just to be sure, in case there was a certain low level range this could trigger in]
Without resorting to any spoilers, this PSA is fairly simple. Whenever you obtain a Templar follower in Diablo III, do not equip him with a new shield. In fact, don’t equip him with anything just to be sure.
Eurogamer is reporting that this issue apparently is restricted to the Demon Hunter class (wouldn’t you know it, my favorite class), but just in case, I wouldn’t recommend trying it with any class if you value your playtime.
If you engage the glitch, you’ll not only be booted out of D3′s servers, but you’ll be unable to re-enter them until Blizzard pushes a fix. An absurd workaround is noted by some users, consisting of creating a new Power User Windows account, and attempting to log in that way.
Blizzard is investigating. As a temporary workaround it suggested players not give the Templar any gear until after they have accepted him as their follower through the user interface pop-up.