Cannot set Terminal Connections

[EC1]

Hello, I have been actively setting terminals using the "Terminal Connections" UNM tool over the past few days, but it has suddenly stopped working. See picture below.

I've tried:
disabling/enabling topology.
Creating a new map project, which worked for about 3 edits of terminals, then the same error occurred again
Closing/opening arcpro

I've been using this tool for several months without issue, but now I cannot edit any terminals. The devices in the picture are subnetwork controllers with terminals. I managed to set them up with terminals/subnetworks but now I cannot edit the terminals. 

What can I do to fix this? Is there something I am missing?
Thanks.

[MikeMillerGIS]

Do you have copies of the line layer in your TOC?  I have seen this when the line layer is added more than once with Def Queries on it.  Same thing might happen if the device layer is added more than once.

[EC1]

Good suggestion, but that doesn't seem to be the case. I've created a new map project and added minimal layers to it, just all of the required UNM layers (once) and a few others:

The strange thing was the terminal connection tool worked in this new map for very short period of time before coming up with the same Terminal Connection error. 😞