There's a big white box over my presentation slides...Why? and how do I fix it?

I’m using Libre Office Impress. When I am working on my slides they look just fine, but when I go to run the slide show a big white block of space with a “pause” icon in the upper left corner appears and I can only see a sliver of the actual slide on the right hand side of the screen. Any suggestions on how to remedy this??

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In addition to @gabix, your problem should not be.
It could be a problem in your file and we can not see it from our location.

Please edit your initial question, there you can upload your file here. Thank you.