Latest update:
On Sunday night at home, I got the idea that maybe two SDNs were conflicting (on backplane, not on same DNet). After I loaded the scan list again through the U2DN, we started to see 78 codes, so I hoped that was the problem. (no network available though - ergo no devices). I came back to site on Monday.
Now I have code 91 on the network. I also tried disconnecting the blue and white out to the field, so nothing is on the network but the 24V, SDN, terminator resistors, and the U2DN. Still I have code 91.
Any more ideas will be appreciated!