"Dimension" saving problem.

1 Dn is a crap file format for models that should never be used because no one [even Adobe Dimenion] supports it

2 it looks like the underlinning error is in your file path

i would test in the following way;

• create a new folder on your desktop [or some location allowed by your network setup]
• open Dimension, create a new model of any kind for this test then export it out to the new folder you just made as; Fbx, Obj and Dn file types
• close\ open dimenion [without saving], create another new scene with at least 1 new model then try to import the 3 models you saved before i.e, import it as each of the Fbx, Obj and Dn files into this new scene