My fault... I created a new PC for my family some time ago and I lost the passwd for my (unique) admin user. I can access with other users, but none of them has admin priviledges.
I tried the solution found on web to enter the system in recovery mode (left shift), do a "mount -rw -o remount /" and then try (as root) to "passwd ", but i always get "Permission denied / password unchanged".
Why - as a root user - cannot I change an admin password?
Probably I did one year ago some system changes/updates to try running Scracth for my kids (running Adobe Air) and modified some passwd settings that I cannot remember :(
Might you please help me?