Creating a new report for this since the ones I've seen all seem to be either closed, or about Windows or Windows in a VM, and I'm still not sure what I'm supposed to do about it.
I opened Creator today and got a message about some plugins (QmlDesigner, QmlProfiler) having errors and that they can't be loaded. The reason was:
Creator worked fine yesterday. The only thing that I can think of that's changed is that I accepted Ubuntu's regular update prompt that pops up, so perhaps something changed on my system.