UniFi Controller, Let’s Encrypt and “This site can’t provide a secure connection”

You cannot use import_cert to import a cert created by Let's Encrypt. That command is instead supposed to complement and finish the new_cert command.

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.