After updating to 5.14.1.1 the jailbreak worked without problems (following the instructions). Installation of KUAL, renameotabin and koreader worked as well.
Update to 5.14.2, then Update_hotfix_watchthis_custom.bin again, followed by reinstallation of KUAL (coplate), everything works as expected.
Thanks a lot for the jailbreak and the help.