Thanks Luis –
I haven’t had any issues with flow stability; I can run it fine with the crossing removed and the results seem as expected.
I tried deleting recreating the crossing from either direction; I get the same error although if I draw it backwards it says ‘Check that the “To:” is set correctly’.
If I have it set up as a pure 2D model it runs fine, which suggests there’s something about connecting the 1D portion of the model that creates the issue. Which is strange to me.