Thank you very much, I have been able to solve the problem. My Office 2016 did not know Arial Nova at all and therefore used a different font (I had used a template created with Office 365 that had Arial Nova set as the default font).

In PowerPoint, I could not figure out which font was actually used. However, when I converted the slides to a pdf, Acrobat shows me that it is the Carlito font. So now if I replace Arial Nova with Carlito, the slides retain their appearance even when I open them in Office 365.