Thanks for Help!
Sorry, but I am not allowed to give out the file.
But the tip with N-Gons has brought me the solution.
I have converted the font and now the export works.

Hi,
I placed a logo on a cup and connected the objects. Everything was displayed correctly, but when I looked at the FBX file again in Windows 3d viewer it was incorrect. Polygons reconnected where there should be no polygons. However, when I import this FBX file back into Cinema 4D, the object looks normal. What could be the reason for this?

I hope you can help me!
Thanks and greetings

Kevin!