I have multiple setups with this scenario so there must be something else wrong here. Have you configured creating new users? Because it looks like it now tries to create a new account on the Mendix side. So double check your user provisioning.
We also had the same issue, with this error:
“ERROR - SAML_SSO: (63/63) Caused by: com.mendix.systemwideinterfaces.core.UserException: Object id: 30680772461474148, validation errors: (member: Name, message: De gebruikersnaam is al in gebruik.)”.
(This error in English: “Username already in use”.)
We resolved this issue (also) by editing the “Just in time provisioning” items, as these settings caused a conflict on the next login after the auto account creation by the SAML module. The conflict was caused as it tried to update the account by provisioning an username that already existed on another account and this throws a validation check on the User entity (being "Username already exists/already in use”) and prevented the next login.