Seems like nobody else don’t work with headphones ever.
Or the answer to my question is so stupid and simple that nobody dares to explain me this so I wouldn’t look like an idiot…
I use an old RME Fireface 800 and do run into this, but am always able to fix it by doing the following:
Double-checking CR is enabled, and the outputs for Phones are correct
Switching back and forth and making sure use phones Channel as preview is set properly
Checking the volume knob for Phones is up sufficiently in Nuendo & my audio interface
I’m sure you’ve done all of these. I hope you’ve been able to figure it out! I created a shortcut sequence to turn Phones preview on and off which made things a bit easier.