BuddyPress.org

The resolution from apeatling (in response to https://trac.buddypress.org/ticket/1356):

If you are allowing @ symbols in usernames, then please add this to your wp-config.php:

define( ‘BP_ENABLE_USERNAME_COMPATIBILITY_MODE’, true );

https://codex.buddypress.org/how-to-guides/changing-internal-configuration-settings/

Worked for me!