I assume the powerpoint itself is enabled in the display profile assigned to your devices?
Display Settings page → Edit display profile assigned to your devices → Enable PowerPoint?
Full Office with PowerPoint itself installed on the PC with Xibo player and those changes added as well - PowerPoint media items show a "Navigation Cancelled" error when running on the Windows Player / PowerPoint brings up an Open Dialog
There isn’t much more that could be changed to stop .pptx from working if they were fine before.
Do any changes were made to the CMS/ player PCs recently?
If it was working and now it isn’t with the same versions, then presumably some changes had to be made somewhere.
Could you explain it to us please, perhaps with a screenshot, are there any log messages in the player status window / CMS log page?