I recently changed the name of a file that I no longer need as it no longer represents anything of my project. (Fortunately I didn't delete it!) Subsequently I opened one of my still-valid corridor files. When I selected the corridor element I found that all I could do is view the properties. When I closed my valid corridor file and then renamed the one file back to what it had been, the corridor becomes workable again. (It occurs to me that it may have worked if I deleted the no-longer-valid reference attachment, since upon detachment the usual case is that the corridor permanently loses all references to anything that attachment had been contributing, though it is obviously not guaranteed.)
This behavior makes sense, to an extent, but there should be a more ready explanation for why the corridor cannot be manipulated.
And the elements referenced by the corridor which cannot be reached should appear in red. And if those references serve as point controls or anything else, those should show up in red too with an explanation for why it's red.