Just recently bought the game with the DLC addon as well but for some reason I get Error Code 300016 on the EU server. Everything works fine on the Asia and America servers. I have no clue as to why but since I’m european I’d much rather get started on the EU server, so I can play with friends. I’ve flushed DNS, repaired it and nothing works. I need help 
Hi Seal : Welcome to the Europe forums 
Error 300016 is a generic connection error… there doesn’t seem to be a solution that will work for everyone 
Blizzard has put some suggested steps here:
There is also this support note :
which will re-direct you to the Connection troubleshooting.
For some people, re-installing the game helped and allowed them to connect to the game.
If all else fail… submit a ticket to have Blizzard take a look at your system and your connection data and see if there is something else they can suggest; you can reach them here: Contact Support - Blizzard Support. Use the Contact Us button at the bottom of the page. It will direct you to a form that you will need to fill-out to submit the ticket… blue-coloured text are links to additional information, procedures, etc… that could be helpful in filling out the form.
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Thread closed as it’s a duplicate topic. Please see the stickied thread Error 395000 / 300016 09/08/2020+ - #12 by Gosan-21774