Moving app icons in panel causes logout and missing dock/panel

When I try moving app icons in the panel or dock (depending on what style desktop I'm using). The screen will flash and dock/panel sometimes disappears. Sometimes it just makes the screen flash. If I hold my click down to drag icon for too long, the system goes to black screen and immediately returns to login screen - usually followed by the dock missing... I can get missing dock back if I go to settings and choose a different look to desktop or re-select the same one I was using before. I would like to be able to move Firefox to be next to Chromium etc.