Could it be that when I install Electrum 1.x with the command "sudo python setup.py install" (just as the "readme:" file says), the libraries installed now in Ubuntu 16.4.1 are more current than those from that time, and because of that, Electrum 1.x can't decode?
Couldn't be, the function used on decoding it doesn't rely on external libraries.
Links:
If there's an issue with the updated libraries, you would likely get a different error.
Since the old version relies on a fixed wordlist unlike the new seed version system, yours could be containing at least one mistyped letter.
Check each your seed phrase if at least one word doesn't match with the old wordlist in "
old_mnemonic.py" link above. (
line 32-1657)