Trying to mount old encrypted home

I strongly recommend that you use the ecryptfs-recover-private utility in these cases.

A full explanation of how to use it is available here.

I finally managed to mount my encrypted home by first adding the passphrase via

sudo ecryptfs-add-passphrase --fnek

Which gets me the following information:

Inserted auth tok with sig [aaaaaaaaaaaaaaaa] into the user session keyring
Inserted auth tok with sig [bbbbbbbbbbbbbbbb] into the user session keyring

There, I remember bbbbbbbbbbbbbbbb and proceed with mounting the associated .Private directory:

sudo mount -t ecryptfs /mnt/oldhome/.ecryptfs/me/.Private /mnt/oldme

A lot of questions pop up, which I all answer with their defaults except for

Enable filename encryption (y/n) [n]: y

and

Filename Encryption Key (FNEK) Signature [aaaaaaaaaaaaaaaa]: bbbbbbbbbbbbbbbb

With bbbbbbbbbbbbbbbb, of course, being the signature I previously remembered. Easy, huh?