Hi,
Could you try disabling all your own custom css and or scrips to see if this resolves the issue. I usually just disable the Master page inheritance for a moment to see if this helps.
If this is the case you might be able to work with the Kentico classes to fix the issue just for Kentico admin pages:
.EditMode .yourcustomclass
or
.DesignMode .yourcustomclass
Greets,
David