Font/Font size change, needs double click

Hello guys,

when I try to choose a font or font size, I click on the symbol and then on the font (Arial Black), but then I need to click on it again. 

It is with every font and the same with the font size.

My solution I tried:

1. This bug doesn't exist in the Dev, so I tried to import the solution in the Prod. But it is still there
2. I checked the HTML code between the DEV and Prod and the only difference I have found is in this
   1.   When I mark this area this HTML Code comes: <div class="ms-crm-Floating-Div" tabindex="0" style="opacity: 0; z-index: 2004; display: block; height: 825px; width: 1924px;"></div> and the only difference between this and the Prod was the "z-index: 2004;" in the Prod and "z-index: 2004;" in the DEV.

None of this worked and for me it feels like a bug, that needs to be worked on in more depth than I can.

Any help is appreciated