BuddyPress.org

bp-core/bp-core-signup.php

Please look into the array_merge function you have used /introduced in BP 1.2.3 here.

Apparently it is appending the illegal_name string each time a user signs up or the validation is performed and then updating that in the wp-sitemeta table.

It is a serious error as the query for update keeps on increasing until it reaches a point where the database starts throwing an error.