Okay thanks, now I know what you mean.
I must have accidently clicked the arrow icon.

@Julian Eisel (Severin):
Sorry, I didn't see your comment until now.
Exporting the file under a different location/different file name doesn't help.

I've found a workaround for this bug:
If I export the disc as PLY file, create a new blender file and import the PLY file there, then I can export the disc as STL file from there.