Error code 401 when trying to do backup

Hi there. For the last couple of weeks I have been unable to backup from Locus to my Google Drive. It was working perfectly until then, but now I just get an error "missing authentication credentials". I have tried logging out of Locus and back in again, and have also rebooted my phone a couple of times but I still get the same problem. Please can you help me, as I feel uncomfortable being able to back my app up.

Thanks

Adam