Thank you for the details, I have moved them to a file, which is not accessible to the public.
I logged in to check, and interestingly if I switch to the “Regular engine” the search seems to be working. The index table however seems to be perfectly fine as well, so I am not sure. I suspect this is either a custom code conflicting or there is a yet unknown bug.
Can you please add temporary FTP access, I would like to debug the query.