Basically, if the Windows username you're using features any characters in this handy list, then you'll need to sign in with another user account, or else create a new one.
Rockstar Support has already released a workaround for the issue, which is caused by Windows usernames with unconventional characters.
But the stats are refusing to update as I complete the final street races and Tonya missions.
I'll be disappointed if the hours I've sunk trying to get 100% aren't updated on Social Club - because that's where I want to do all my bragging!
The current setup file available for download occupies 211.8 MB on disk.
The software can also be called "Rockstar Games Social Club".