Even on my mum pc, all the sort of weird problems, and she doesn't even know
the password of the laptop....
What problems?
With a physical access to the computer, there are many ways to get a root
terminal, without any password. For instance using 'chroot' from any live
GNU/Linux system. Once you have a root terminal on the installed system,
'passwd' allows you to redefine the password of any user.
(I commented GRUB_DISABLE_RECOVERY="true" in /etc/default/grub and ran 'sudo
update-grub', to have the option to directly boot in a root terminal from
GRUB's menu.)
