I had the same problem trying to upgrade from postgresql 11 to 13. I got the same "ee key too small" error.
pg_upgradecluster exited without creating a working upgrade and without giving an error message. It did not appear to fail. I dropped the non-working pg 13 cluster and then ran: sudo make-ssl-cert generate-default-snakeoil --force-overwrite Next, I ran pg_upgradecluster and it worked seemlessly. pg_upgradecluster needs a more obvious error message when it is not able to complete properly.