In Stat 5.7.x, you need to make sure that you are not manually entering / copying the USER DN information for Stat user under Maintenance | Security Settings | User Maintenance | USER DN column.
The manual modification of USER DN field that worked fine in previous versions of Stat will not work in Stat 5.7.x.
For new users that you create in Stat 5.7.x, the proper way is to first go to:
Maintenance | General | System Maintenance | Other Options
On bottom left corner check the box for “Enable LDAP”.
Then go to:
Maintenance | Security Settings | LDAP connect
Mouse click on the row for the LDAP ENV that you are using, make sure values under Host, port, Base DN and Template DN are correct, then check the box under Append Base DN column, then mouse click on "Assign to users" tab on top then mouse click on the row for the user id that is having this issue then mouse click on "Update User DN" button then mouse click on Apply button.
Now in the same window if you look under "User DN" column for this user, the information displayed here for this user should be the same format (CN=..\, .......,OU=...,OU=..., etc) as other users that can successfully login using LDAP and the box under "ENABLE LDAP" should be checked for this user.