From The Document Foundation Wiki
LibreOffice 5.2 and older
- close LibreOffice including "QuickStart" in system tray
- rename the user profile folder from "user" to "user-old", locations for your operating system are documented below
- restart LibreOffice and a new user profile will be generated
- Test and see if the problem is gone.
- If the problem persists, you can restore the previous profile because it was not the cause of the problem. To restore the previous profile, delete the new folder and give the old one its former name.
- If the problem is solved, one or more files on your profile are corrupted.