Hello,
My situation is rather complicated.
I just tried upgrading LibreOffice v7 to 7.3 on my SO's Ubuntu computer, classically running the dpkg command as per the book. This failed, and after a nonconclusive attempt to deinstall/reinstall I launched LibreOffice from the terminal and detected a file named libnsso.so was not with the right version.
Having LibreOffice 7.3 running on my own Debian 11 machine, I got the extremely bad idea to replace the old Ubuntu file with my more recent one, which of course I did with silly sudo commands.
Since then the Ubuntu machine just refuses its user password, not even opening a prompt for it (there is a red mention talking about encryption failure instead).

I managed to switch boot to an old USB live key with Debian 10 on it; I now see the internal disks.
I thought it'd be easy to retrieve the old libnss3.so file and put it back in place; alas, my SO's user files are encrypted (on a separate disk) with ecryptfs.

I do have her passphrase (and password) but when I try the command "mount--t ecryptfs /..." it fails with the message 'can't find in /etc/fstab', which I interpret as normal, because I'm running from the USB live disk with a probably minimal fstab (a command that I do NOT master anyway -and in addition the volume is already mounted! But I don't know ecryptfs actually...)

What can I do?

Is my idea of recuperating the old file the best one? (wat would be the right ecryptfs command?)
Ideally I'd like to repair the system.
Otherwise I at least would like to retrieve the user disk.

If all goes wrong I also have backups created with 'Backintime' (https://github.com/bit-team/backintime) so I presume in theory I could reinstall the whole system (she has LOTS of apps...) and then retrieve all user files from a backup, but for me very low-level user (as you could definitely see) this means days of work :-(


Thank you!!
Hervé

When mounting filesystem which is not defined in fstab you have to specify mountpoint explicitly: mount -t type <device> <mountpoint>

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If you can chroot into the damaged system you should be able to apt-get reinstall libnss3, but if not, download the .deb package from which to copy the file you replaced.

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Thank you all for your advice!
I thnk we can close this topic; in the end I just managed to revert the system to its previous state thanks to the 'Timeshift' app that I fortunately had installed. While daring (for me) the process went very fluid and safe in fact :-)
Thank you again for your very precise insight, that I defintely will remember!
Hervé