NetScaler Gateway for MAC is not connecting correctly: Protocoll shows "AGAS: Failed to initialise ssl vpn plugin"

Found the solution: In the mac OS high Sierra there is an extra security setting that blocks some applications for some reasons and that had blocked citrix.

You can find it in the Systems Settings -> Security. And there at the bottom of the window is a hint that shows that some software is blocked.

There is also a button to show the blocked software. You have to find citrix in the list and remove it. After that you have to restart mac.