Right, pop-up might be ‘behind’ Xibo, because windows client is set to always be on top - although you can press ‘i’ on your keyboard that will bring the status window and then client will no longer be always on top - meaning you can alt+tab to different application.
That being said, perhaps have a look here PowerPoint media items show a "Navigation Cancelled" error when running on the Windows Player / PowerPoint brings up an Open Dialog
Additionally if you’re using Windows 10, please check display settings and make sure that ‘Change the size of text, apps and other items’ is set to 100%.
In some cases, it could also be good to specify monitor resolution in Display settings -> edit display profile assigned to your device -> location (in Xibo CMS).