My team mates worked hard to improve release the new August 2018 preupgrade.jar for upgrades to 18c and 18.104.22.168. You can download both from myOracle Support.
New August 2018 preupgrade.jar for upgrades to 18c and 22.214.171.124
As usual, MOS Note: 884522.1 gives you access to the most recent versions of the
preupgrade.jar tools (and of course the previous
utlu112i.sql scripts). One of the major changes is the inclusion for a check for the XMLCONCAT synonym.
Please find the most recent versions here and exchange your current version of
preupgrade.jar with the newer ones.