In includes/functions_privmsgs.php:

When a user has a “If sent to my default usergroup” rule, an array to string conversion warning is triggered when check_rule is called because user_in_group is defined as an array (on line 493), but the RULE_TO_GROUP check at line 311 uses this value as a string.

Changing the definition from an array to a string performs the equivalent operation without an implicit conversion.