Because the upgrader has received so much work since it's last public release (2019 July to 2020 January/today), and you are the first to report any errors with it, I would like to attempt to reproduce and debug your error:
What OS ar you upgrading on? Windows / Linux / other?
What are your PHP and SQL versions?
What version of UBB.threads are yo upgrading from?
What version of UBB.threads are yo upgrading to?
