The hosting company is blowing smoke without looking into your particular issue, but I doubt they'd be able to help you anyway. Indexing is not the problem, the problem Kuhyay is pointing out has been going on for quite awhile, I noticed the issue at least a year ago. It's Google Search-specific (doesn't replicate using Bing's search page). I do SEO and I've never seen this particular issue. Googling "achachairu tropicalfruitforum" and clicking on the link leads to the 404 page not found return, but if you copy the URL in the address bar (
http://tropicalfruitforum.com/index.php?topic=11629.50) and paste it back directly into the address bar and click enter,
the page loads! So, the exact same link works differently depending on how you access it. My guess is Google is injecting something into the request when a link is clicked that the SMF forum software doesn't like. Something weird is going on, I could probably figure it out with a few hours of digging, but I'm spread thin right now.
While I'm at it, you really need to put an SSL cert on this site and start serving it securely (https). With Let'sEncrypt, it's free and painless to implement. Sites still serving non-secure (http) content are penalized by Google in the SERPs (not that it's a big issue on your free site, but it's probably costing you new members). If your hosting provider doesn't offer the ability to add a Let'sEncrypt SSL cert for free, it shouldn't be more than a few dollars a year. If they are charging more than that, I would find a new hosting company....