@Dev, Installed the new v8.4.2 client - but where did my previous Espers go?
Importing my private keys (dumped in the previous version) gives the error: Invalid private key (code -5).
Did you backup your wallet.dat?
whenever you reinstall a client you need to backup your wallet.dat in you %appdata% however in this case the update did not have anything to do with the wallet.dat
i would recommend immediately setting up backups offline in secure places if possible.
however for your issue, i would try to recover your previous client to make sure the esp are still there and you did not accidentally delete them. then reinstall the new client again.