You cannot use
import_cert to import a cert created by Let's Encrypt. That command is instead supposed to complement and finish the
However, if you did accidentally do this, in your
data folder (which on my Linux box was
/usr/lib/unifi/data) there should hopefully be a
keystore.before_import file that you can copy back onto
keystore and then just restart the unifi service with
sudo service unifi restart.