A while ago, mintupdate stopped working for me, claiming it was unable to grant execute permissions to the non-admin user. Investigating a bit further, I found that my regular user “hoover” had gone missing (for whatever reason) from the “admin” group; this can be fixed by re-adding your regular user to this group.

Simply open a shell and type

sudo usermod -G admin <your_username>

and mintupdate should work again.