Exported a noisy NEF a JPG (sRGB),
used this JPG to produce a 16bit TIFF,
which then I could open, process and save in DFine2 without problem.
PL5.1.2.4700 // DFine 5.03.101 (v3.3) *)
*) re-installed Nik Collection v3.3
→ Issues with Nik4 - #8 by Wolfgang