Hi,
today I restarted my computer and found that the gnome surface is not starting properly. I have a black screen with trash and home directory icon, which work and skype is opening but the 'start' field is not showing up.
Entering the console via Alt+Strg+F2 I can access everything.
One possible reason I thought of was that I tried to deinstall python2.7 in order to install python3. I know now that this is idiotic and by doing that I probably deinstalled something important. Nevertheless, as far as I can see, python2.7 still works and I don't understand what might be the issue? I already reinstalled python, but it said that there is nothing to do.
I also tried
yum -y groups install "GNOME Desktop"
but it says that there are no packages in any group available for installing or updating.
Can you please help me?
I have a Centos 7, 64 kernel system.

Try yum distro-sync.

To do this, I have to use --skip-broken since there are unresolved dependencies. If I do so, it has to skip nux-desktop but then the panel is back. Nevertheless, this is only if I enter it as a root user. How can I keep it also for 'normal' user?
Thanks!

You enter what as root user? GNOME? Don't do this. After logging once into GUI as root you may get all sorts of permissions problems as regular user afterwards. Quite often, it manifests itself as .ICEAuthority permission/ownership problem. Google for it. Or search here on LQ.

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To do 'yum disto-sync' I have to be root user. Otherwise it doesn't work. If I then enter the GUI, I have the panel and everything else back. But if I then switch to normal user, it is gone and I get black screen with some icons.

Open a terminal as a normal user and su to root. You should never login to a GUI as root, as mentioned earlier.

Ok but what can I do now?

Obviously, "yum distro-sync" helps but not for the user. How can I apply this for the user?
Just to be clear:
I login to the user account: black screen and some icons appear but no panel.
Then I enter a shell via "Strg+Alt+F2",
login>> user
Password>> password
user@linux>> su
root@linux>> yum distro-sync
(if I would do: root@linux>> startx #gives panel, otherwise...)
root@linux>> exit
user@linux>> startx #->Still black screen and some icons.

Creating another user gives me the GUI for that user.
But it would be nice to restore the GUI for that first user still.

As root do
Then start X as regular user.

yum distro-sync is for the OS, not the user. Those command are run as root and have nothing to do with any user.

I could solve it by creating another user (adduser) and copying
.local/share/gnome-settings-daemon/ and
.local/share/gnome-shell/
from its home directory to the home directory of the old user.

This was not the problem. It was the user's directory, not root.

I know but I did not understand why the configs did not stay for the users. I've understood now, that the reason of the error were local configs and yum distro-sync does not touch them.

Thanks

Exactly. User configurations are always in /home and have nothing to do with the operating system, only the user. The command you ran was for the package management system and was for the OS only.