I have been using wpForo on my private membership website for quite some time. Never had an issue with "search" results until recently.
Now, even when searching for a single word that I know is in almost all posts, using the "Search Entire Posts" method, it always returns zero search results and the message "Posts not found."
(Attached screenshot showing what a basic search looks like with this message)
My membership and forum is targeted at helping people with their photo collections, so searching for the single word "photos" should pull up nearly every post when searching using "Search Entire Posts."
However, of note, when searching using "Find topics by tags", then I DO see results just fine. I tried other search type options and the only other one that does bring search results is "Find Topics Started by User." All the others also show no results.
I copied my website to a staging area on my Cloudways host, deactivated every single plugin except the wpForo plugin and wpForo addons, and still no search results with "Search Entire Posts."
I turned off Varnish (server-side caching) and still no search results with "Search Entire Posts."
I changed my Wordpress theme from Divi to a stock Wordpress theme and still no search results with "Search Entire Posts."
I then deactivated all wpForo addon plugins, so only the wpForo plugin and default wordpress theme were running, and still no search results with "Search Entire Posts."
I'm sorry, I can't share a link to my forum here in this forum since mine is a private group. I could share access to my forum in my staging area by sharing credentials through DM or email etc. with support.
I was hoping this amount of information might be helpful (enough) if support in here has already helped someone in the past with a similar situation.
I think this is a database MariaDB 10.1 bug with "full text" index. First check if there are any db error in Dashboard > Forums > Tools > Tables Tab. If there isn't then try to delete "full text" indexes in wp_wpforo_table then create them again for the title and body fields. Use phpMyAdmin database manage plugin or the one in your hosting cPanel.