I think using JDownloader (java based software) blocks access other applications as root, but not only then ...once I rebooted my computer and I couldn't open any application as root right away. In this situation I can only open console and log-in as root (with "su" command), but when I try to do "yast --qt" as root it gives me an error:

No protocol specified
QXcbConnection: Could not connect to display :0
/sbin/yast: line 440:  2075 Terminated               $y2ccbin $Y2UI_ARGS "$@"

I can access YaST only in the text mode within the console (with just "yast" command). I think this problem occurred since when I installed JDownloader and updated it with new feature that makes it able to join split files. I think that because earlier I did not have this problem.