Was the installation of 0.18 where it occurred first on a plain vanilla system or did you have another FreeCAD version installed before? If the latter you can try to reset the configuration with the button in the preferences. Another possibility is to delete your config files
or to move them in case you want to restore them.
Another possibility to find the problem is to start FreeCAD with the additional commandline parameter -l (small letter L), which creates a logfile in the directory mentioned above.