Delete some files using SSH:
- Login to SSH using root / [password of admin user]
- cd /
- rm .syno/patch/*
Delete some applications using package manager:
- Login to admin http://SERVER:5000
- Review the installed applications, stop or uninstall not used application(s)
- This will free up some space in root folder (/), check available space using df -h
Sindicato ESP. 
Mate you are a genius! This workaround fixed a problem on my Synology box. I was unable to log in and could not retrieve my amassed Photo collection of my daughter. You saved my bacon! Thanyou so much!
you saved my ass !!!!!!!!
Grand its worked and saved a lot of time for me.