That error says that it is trying to find an invalid offset to the array, which means key to an array is missing, which also means it cannot find $settings['theme'] in your system. That settings in your system should be
Doesn't matter what theme he runs at, and disable user theme in your user fields if any. This will force all users run system default theme. When all done, then you may reinstall back user theme user field.
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Need help?, Having trouble? • View our Documentation for Guides, Standards and Functions • Name and Organize your Topics and Content correctly in the corresponding Forums for best support results • Attaching Log Files and Screenshots when reporting issues will help • Provide with an URL to live example if one exists • Please read the How to Report an Error post • Please read and comply with the Code of Conduct
Need help?, Having trouble? • View our Documentation for Guides, Standards and Functions • Name and Organize your Topics and Content correctly in the corresponding Forums for best support results • Attaching Log Files and Screenshots when reporting issues will help • Provide with an URL to live example if one exists • Please read the How to Report an Error post • Please read and comply with the Code of Conduct