FlashChat is severely outdated, it was coded in the days of PHP4 and very early builds of UBB.threads v7; as such, it shouldn't run under newer sets of UBB.threads coding; I'm not even sure if FlachChat would even run under PHP5.
This is
not a "UBB" issue, it's instead an outdated script that the author hasn't done the groundwork to ensure compatibility with current releases, modern coding, or with the scripts it used to integrate with.
I am working on a wrapper to use IRC in the UBB; I can release an early beta of that coding if chat is something you would need for your community (
example).