Hi Sellden and welcome to the club
I don't know for sure, because I never had a Motu interface, but it sounds to me as if the Motu interface automatically switches to "word clock SLAVE" when it detects an incoming digital audio signal. A number of audio interfaces do that.
If, for example, you had the D8B set to be word clock SLAVE, and the Motu as word clock MASTER, but the Motu detects a signal coming in at a digital audio input, it might "think" that it has to change to clock "slave" in order to properly sync to that... and by doing so, creating a situation where you have two devices that want to work as "word clock slave" (but no word clock MASTER), and then audio will no longer pass. When they're both "slave" and try to sync to each other, an "auto-sync" device would start to kinda freak out and switch through sample rates constantly, to try to figure out how it could get the sync working.
Long story short - what happens when you set the D8B to act as "word clock MASTER" and the Motu as "word clock SLAVE"? Does that work, and pass audio between the D8B and Motu over the digital/ADAT cables?
The D8B doesn't do any auto-detection for it's master/slave settings, so this configuration might very well help to pass audio.
If reversing the Master/Slave settings DOES work (i.e. D8B Master, Motu Slave), then I think it would be best to still try if there's a setting in the Motu, to turn "auto-sync" off, and have the Motu act as the Word Clock MASTER (and the D8B as slave - but you need to get it stable the other way around first, to identify if word clock sync is the problem).
With the Motu being newer, the Word Clock is likely FAR superior to that of the D8B - even if an Apogee Word Clock card is used in the D8B. So, Motu-Master/D8B-Slave would be preferable, IMO, if you can get it to work that way.
(To support that claim, when I added a Lucid GENx192 Master Clock to my setup, the audio quality of my UA Apollo and PreSonus Quantum didn't improve (they're already great sounding), but I noticed a clear difference with the D8B - the stereo image and depth clearly benefited from a decent Master Clock).
I hope this helps to put your new setup on the right track. Post back how it went!
Best of luck and stay healthy!