This seems to be a minor error with the phpBB.com/Community website's search function. When searching through a user's posts, every search request beyond starting from post 19985 results in an error. This means that some of the posts by older members with lots of posts cannot be searched with the "Search user's posts" function.

Example:
https://www.phpbb.com/community/search.php?author_id=191594&start=19985
This link is as far back one can search a user's posts before getting an error. Advancing the page or the number, even just by one results in: "Sorry, search could not be performed. More information about this failure has been logged in the error log."

AmigoJack gave me information and took a screenshot: https://www.phpbb.com/community/viewtopic.php?p=15590891#p15590891