When a friend upgraded his notebook from Plasma 4 to 5, KWin did not start initially. It turned out that the problem was an "export KDEWM=kwin_gles" in his .profile; and that binary was gone with KWin 5.x It would be nice if startkde would validate the existence of the executable given in $KDEWM before using it, so that such long-forgotten adjustments don't confuse other upgraders. Reproducible: Always
startkde simply sends it to ksmserver, I think ksmserver should do the validating based on whether it can/can't launch the given app. Reassigning.
None of this is relevant anymore, thankfully. :)