If you are using extbase yourself you dont have to call makeInstance for your objectManager, it's already there ($this->objectManager
).
anyway, you should inject this repository (see my answer here: TYPO3 - Call another repository)
Clear the Cache after the Injection.
You maybe have to disable the recordtype extbase sets for its FrontendUser:
config.tx_extbase.persistence.classes.Tx_Extbase_Domain_Model_FrontendUser.mapping.recordType >