Thank you so much!
I tried the safe mode first but the same issues occurred, so I followed the second method. In the new account I created, Mail at first opens up but when I start to configure an account it tells me the app has no internet connection. Safari as well shows the charging bar under the url stopping almost at the beginning and never completing and the app store shows always its empty window.
But the internet connection is working, because I can go on the internet through Chrome without any problem.
So I guess there's a connection issue between those apps and the Internet, any ideas on how to fix it?
Thank you so much again for your reply