PhpBB's search is totally broken anyway with its non-configurable word substitution and rejection of short words. I'm not really surprised that it chokes on some input.
After my testing I conclude it doesn't seem to be a real vulnerability, although to be really sure an actual infosec professional should be asked. It appears that user input at the point of crash is used as https://www.postgresql.org/docs/current ... PE-TSQUERY rather than a plain string that could cause harm.
Do I understand the linked page correctly that this is the mostly non-functioning word substitution thing Graf was talking about which often makes forum search such a major pain in the ass?