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How did you perform the upgrade? Did you follow the instructions? Or did you use a third party service like Softaculous? The latter may have left files behind that must not exist for 3.9 to work. In which case ensure that the source code folder contains only Moodle 3.9.1 files, e.g. by moving the existing files to a backup folder and replace them with the official Moodle download, and copying config.php across.