On my Synology NAS box I got this error message when trying to login to the admin console (synology cannot login to the system because the disk space is full) and was not able to login.
I could still telnet to the box so issued a reboot command.
After the device had rebooted still could not login so after a bit of hunting issued these commands.
- Login to SSH using root / [password of admin user]
- cd /
- rm .syno/patch/*
It looks like a failed upgrade.