Just to share how I got around this:
By deleting 1 front panel item at a time I found that one single control was causing PaneRelief to crash; an XY graph.
Setting it temporarily to not scale and replacing it with a standard XY graph (the one I had had some colours set to transparent etc) was enough to avoid having PaneRelief crash LabVIEW, but it would now just present a timeout error:
I found a way arund this too though: the VI in question was member of a DQMH lvlib that probably added a lot of complexity for PaneRelief. With a copy saved as a non-member it worked: I could replace the graph, edit the splitters with PaneRelief without the timeout error (even setting the size to 0), then copy back the original graph replacing the temporary one, and finally move the copy back into the lvlib and swap it with the original. Voila! What a Relief... 😉
I probably have to repeat this whole ordeal if I ever need to readjust the splitters in that VI with PaneRelief though 😮