Hello Marc,
I believe that due the blocking state on STP we will miss some data to be able to create connection between those two devices.
For basic troubleshooting please check in DB:
( check for those two nodes)
1) SELECT * FROM NodeL2Connections where NodeId = X
2) SELECT * FROM NodeCiscoCdpEntries where NodeId = X
Add 1) - it stores data from CAM table, due blocking state on STP I think it won't have stored MAC address of the another device.
Add 2) - if you have CDP enabled, we might have the needed data, because CDP sends/recevies managment packets.
Best regards,
John