Re: Fatal error: Class ’Bp_Activity_Tags_Widget’
It looks like you are trying to use this plugin – https://buddypress.org/community/groups/buddypress-activity-tags/ – because `Bp_Activity_Tags_Widget` is not in BuddyPress itself.
Try deactivating the plugin or, if you’re unable to log in to the admin, remove `bp-activity-tags` folder from the `wp-content/plugins` directory.
And maybe remove the short-code or widget corresponding to this plugin from your theme as well.