If you have hundreds of Redmine projects, and someone has created them from other projects, chances are you will have a default assignee that doesn’t belong on all those projects. Login to the Redmine database and update the projects table accordingly.

mysql> select id,name,default_assignee_id from projects where default_assignee_id=6;
+----+---------------+---------------------+
| id | name          | default_assignee_id |
+----+---------------+---------------------+
| 14 | Project1      |                   6 |
| 55 | SomeProject2  |                   6 |
| 56 | Proj3         |                   6 |
(many rows omitted)
+----+---------------+---------------------+
57 rows in set (0.01 sec)

mysql> update projecst set default_assignee_id=NULL where default_assignee_id=6;