Did you import the user into Mendix?
If you want to enable LDAP authentication each user must also be a Mendix user. You also need to tell Mendix which user uses the LDAP authentication and which one not.
Thank you for your response. In the LDAP settings there is a LDAP type field that has the following option.
"Authenticate and create: This will not synchronize users, but if a user that is unknown in Mendix logs in using a valid LDAP authentication, a Mendix user will be created and the user info will be requested from LDAP at that moment."
This is the option we are using so if the user is able to authenticate it should create the system user, however it does not, even though I know the credentials are correct as I used them with the "Test Authentication" option popup.
After some debugging we narrowed down the issue to special characters in our group name. The synchronization worked as expected for a group without a '#' in the name but it did not work for one with a '#'.