While trying to participate in the Necromancer Beta Test in Diablo 3: Reaper of Souls, you may experience a few errors. The most well known is Code 300016, an error that appears when trying to establish a connection to the beta server and is accompanied by the note, "There was a problem logging in to Diablo III".
To solve this problem, you need to switch your beta client over to 32-bit. This is achieved by clicking Options on the top right of the Diablo 3 launch panel, clicking Game Settings, and then checking the box labeled 'Launch 32-bit client (instead of 64-bit).
We were personally affected by this issue and this setting changed solved our problems.
Enjoy the beta!