do the users have “Primary e-mail address” set in UCS?
I had a similar problem and found out that OpenProject has ‘mailPrimaryAddress’ as requirement for user accounts and will fail to log in a user when the account don’t have the email account set in LDAP.
To set the “Primary e-mail address” you need a “Mail” domain in the Domain section (in blue).
Alternatively, you could use the ‘admin’ credentials in OpenProject (settings->LDAP Auth section) to set ‘mail’ instead of ‘mailPrimaryAddress’ for example, that way you don’t need to create a “Mail” domain in UCS. Although you may need to configure email relay in UCS to use the password recovery features from ProjectOpen, but I have not tested that.