I had the exact same problem and just solved it after a lot of head banging!!
Just check that in control panel the credential manager is running and that there is an entry in the for the PC\homegroupuser$.
If your credential manger is disabled then re-enable through gpedit.msc and changing this setting:
Computer Config\Windows Settings\Security Settings\Local Policies\Security Options\Network access: Do not allow storage of passwords and credentials for network authentication
Please ensure that this policy is disabled.
Once enabled leave and re-join the homegroup and then reboot.
Hope it works for you.