If your website won't load after updating to BuddyPress 12.1.1 it is likely a conflict with bbPress or another BuddyPress compatible plugin. Install the BP Classic plugin to fix this issue.

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Enabling the "BuddyPress Legacy" Template Pack

This theme requires that you enable the BuddyPress Legacy template pack. To do this go to Settings > BuddyPress > Options and from the Template Pack dropdown menu select "BuddyPress Legacy" and click Save Settings.