When you tried to upgrade did you see EXACTLY the same error message as Roy attached at the start of this thread? If not, then you do not need to use this fix. It will do nothing for you. See my final sentence in the post before yours. (deleted – post moved)
Auch my table prefix is not KT but HS so my upgrade failed.
References here to “KT” are not table prefixes, so your concern is not necessary. Kiwitrees code does not care what table prefix you use, it will always work.
Regarding your error messages, they are common with this upgrade. They are caused by the stored session details from before you upgraded. They do not mean the upgrade failed. Logout, close your browser and log back in. You will not see those messages again.
EDIT…. I have added a note about this to the upgrading instructions for this release.