Just noticed the probable problem buried there: You said "The external HD was encrypted some months ago, with an older version of Ubuntu."
Newer versions of eCryptfs do not always work on data encrypted with older versions. It's not fully backwards-compatible. You should decrypt the data using the original version that encrypted it.
Or, it's possible you're not using the right key for the filename encryption (fnek), double-check the key and look into the mount options ecryptfs_fnek_sig=(fnek_sig)
and even ecryptfs_sig=(fekek_sig)
.