Thank you @sbohmer, your confirmation about the 2.0+ URL pointed me to the solution. Previously I had only changed the value of the URL, but had not created a new server. I realised, that there was a certificate of ios.meraki.com in the properties of my old server config, which did not change when I put in a different URL.
By creating a new server with the 2.0+ URL, I saw the certificate *.meraki.com in the properties.
Afterwards it worked immediately.
So my fault was to rewrite the registration url of an existing server, not creating a new server with registration url.
Hope this helps!