Since Plasma 5.10.4-2 update, moving panel on the left or right causes panel to freeze before attaching to the side of the screen, after half of minute plasmashell crashes. After relaunching plasmashell, panel is back in the old place. So basically panel up or down works ok, right or left causes crashes (in moment of saving config, which fails).
*** This bug has been marked as a duplicate of bug 384214 ***