Hi Maria,
Yes, the unregistered service user must not have a candidate object assigned.
These SLG1 errors may refer to wrong SICF services. So that one service or more tries to be called with this user but shouldn't (or the logon procedure in the SICF service is set).
Under IMG step: Technical Settings -> User Interfaces -> Candidate -> Front-End Candidate -> Specify E-Recruiting services (Web-Dynpro))
the documentation explains how to set it up.
Regards,
Nicole