Bug 395497 keeps bugging me. The only temporary solution is to delete
~/.local/share/kxmlgui5/okular/ or its content, or reset toolbar to defaults, which to me is not a solution at all: I prefer to have a customized toolbar and odd menu titles (than I can hide) than no custom toolbar.
This is triggered at random periods by editing changing/adding toolbar buttons. Unlike what is said by some user under the bug report, both rc files at that path are recreated.