GRUB error

I've set a password for GRUB in my desktop machine and now i can't boot Trisquel and i get the following error:

error: png: chunk size error.
alloc magic is broken at 0x1f706670
Aborted. Press any key to exit.

If i press any key i'll just keep getting the same message.

Before this happened i had edited /etc/grub.d/00_header and placed at the bottom of the file the following:

cat << EOF

set superusers="MYNAME"

password_pbkdf2 MYNAME grub.pbkdf2.sha512.10000.XXXXXXXXXXX....

password_pbkdf2 myname grub.pbkdf2.sha512.10000.XXXXXXXXXXX....

EOF

Note: I made two entries, one in uppercase (MYNAME) and another in lowercase (myname) with the same password hash so that i could type "MYNAME" or "myname" at the GRUB boot screen. (I've abbreviated/faked the hash here; it's just an example!)

Then i edited /etc/grub.d/10_linux and added --users at the line printf "menuentry '${title}' ${CLASS} --users {\n" "${os}" "${version}", like explained here: https://help.ubuntu.com/community/Grub2/Passwords Then i did update-grub

I must say that my /boot, / and /usr partitions are set to mount read-only in fstab and that all subfolders in /var are set to mount as tmpfs (except /var/tmp, which is really a symlink to /tmp - also set to mount as tmpfs)

I've chose to made such partitions mount as tmpfs because i intend to later, when i have everything just the way i want, do a dd and copy everything onto a microSD - it's supposed to be a quasi-stateless proxy server and i want to reduce all writes to it to preserve its lifespan. Could that have any side-effect and mess with GRUB password?!

Could i use a liveCD to edit again those /etc/grub.d/ files and do a update-grub on the installed system? (Please tell me how exactly :/)

Thanks.