non overlapping endpoints..
Check each segment start/end coordinates for position. If not exactly the same (~0.01mm error is allowed I think), 3D viewer will complain.
Also, your mousebites will be milled away if the yellow lines are the edge.cut lines..
It needs to look like this:
I suggest drawing those edge.cuts lines on a 1.0 mm grid for good results - or until you know what you're doing.