Thank you Richard George!

2 step authentication was indeed the problem. So I generated a specific password for Gmail and now it works fine.

If you're reinstalling, you may need to generate a new app specific password in gmail - part of the 2-factor authentication system.
https://www.lifewire.com/unlock-gmail-for-a-new-email-program-or-service-1171974

Can you do SSL instead of TLS? It looks like it is needed in this doc.

Yes, I also tried "Sender Address" my gmail address. I did test it but I get a error. After a tick on the test button I get a window with a "send to" field. I enter my gmail address and tick send now. After that I receive a error with "Authentication failed using username '<address>@gmail.com' and password '**************'. I checked password and mail address.

If you're using Gmail, your sender should be your Gmail e-mail address or you risk the e-mail getting rewritten by GMail or refused. Have you tried sending a test e-mail?