I have the issue again that Splitscreens are not applicable to my slideshow that already has about 300 Slides. The element on the right side is gryed out and can’t be dropped to the timeline or storyboard.
Sometimes a message appears and says “Can not be used with current stage format”.
Only if i start a new slideshow from scratch, than the snippets can be used.
And ff i go to finder an look for the storage folder than it is possible to drop them to the storyboard.
So i found a solution.
I am using a resolution of 1920 x 1200 because this is the resolution of the projector for the diashow. And there is just no snippet made for that. There is one for SD that has 1440x1080.
Why da hell not 
? In the past time it always work and it would also work with my previous workaround. 
So i made a copy of the snippets and change them to 1920x1200 and also modified the mask. Now the snippets for this are usable.
So the FM support gave me the final solution. When savin the snippet the “universal” checkbox must be enabled.
In my opinion this should be already activated by default for evey snippet.