One cause is your home folder permissions are wrong. Just
sudo chmod 775 /home/jimbo
Replace your user home folder name for jimbo.
This annoyed me to no end till I found this.
Edit (11-12-2020): There is another obscure cause of this. No matter what I tried I constantly was being prompted for a password even though the permissions were correct as described above.
My final solution was to backup the data in the account and to delete it, reboot the proxmox lxc container and then add the account back. This fixed it.