The topic was closed because the original issue had been resolved.
Error 300016 is a generic error and can be caused by many things… heck, it even pops up when you play in Starter Edition, kill the Skeleton King and then try to purchase Diablo III from within the game…
There’s been a few cases on Europe Tech Support forums (English, German, Spanish, French and probably others). In addition to the reply of Drakuloth on this forum, there is this reply by Blizzard on the Tech Support Spanish Europe forums. There were no replies by Blizzard on the Europe English and French forums.
Have you tried starting a game in another region ?
Have you tried the Connection troubleshooting as suggested in the support note for Error 300016 ?
What other troubleshooting have you done ?
You can submit a ticket – some issues are just too complex for solving on the forums… it may be that you need some CS person to sit down with your computer system configuration data and your connection data to figure out what is causing the issue in your particular case.