When I press "clear history" button, the history is not deleted. Also not from the context-menu-entry. What works: open qdbusmanager and call the dbus clearHistory function from there.
Please confirm if this is the same as cause as 336053
It seems so (because editing the plasmashellrc file solved it), although I do not remember clicking some confirm dialog.
*** This bug has been marked as a duplicate of bug 336053 ***