Update Camera Raw plugin in Photoshop CC 2015.5.1 to ACR 9.6.1.
Thanks, but that isn't the issue. I should have been more specific on the ACR version currently used, it is 188.8.131.521.
I also need to clarify that it is possible to open the DNG as a Smart Object in Photoshop. However, when I save it uses my External Editing File Naming preference but the new file is not imported in Lightroom and while saving I get an error message in Dutch which is my application language:
FOUT 2: photoshop is niet gedefinieerd
I also get the message if I close without saving.
After a chat with Adobe Support all problems were solved:
Apparently, when using an external source for your photos, which is the case for me to save space on my local drive and for security and backup reasons, sometime that problems I had seem to happen.
The solution was fairly simple: disconnect the external source, import a random photo to LR by using the "Add" import function, open it up in Photoshop by using "edit in". This worked flawlessly and afterwards when reconnecting my main source for the photo's I have, I could again open DNG files in photoshop by using the "edit in" function of LR.
Thank you Adobe Support!