This is a new behavior as of yesterday. I am on fedora kde 23, and I believe that it just upgraded to 5.5.3 prior to this behavior:

IT every now and then (have not discovered a pattern yet) will revert back to air desktop theme (I am using breeze right now). When this happens, it locks up. At least everything on the panel does. I have not tested out the desktop, or folder view widget. I CAN, however, alt-tab and pull up yakuake to reset plasma.

Any ideas, or should I create a bug?

Actually, it gets stuck when it is trying to show a popup message.