I just ran into the same issue (long time since I used qtcreator last) and removing QtProject/ and QtProject.conf fixed the issue. However I also don't want to loose my settings so I started fiddling around, and it turns out for me the error goes away if I clear the list of recent projects and manually select the project again. Thereafter I can just select that project from recent projects without any problems. Does not seem like the real source of the problem to me, but anyway it worked for me and maybe it's useful to anyone else as well.
