Hi, I've tried some of the solutions that Robert, Sophy and Anton so graciously posted for this problem. I thank all of you for all your help. But I still have the problem. I am using MySQL 5.6. and cannot upgrade to 5.7 because my host (GoDaddy) uses the earlier version and I'm on shared Wordpress hosting. I use extended layout. I tried the indices solution that Anton kindly posted but it didn't fix the problem. I would be grateful for any other possible solutions.
This is not a wpForo issue, and cannot be fixed by wpForo side.
Please read Robert's answer: https://wpforo.com/community/how-to-and-troubleshooting-2/topic-pages-disappeared/#post-26309
I'd also suggest you read the following support topic: https://wpforo.com/community/how-to-and-troubleshooting/missing-pagination-for-guests-in-extended-layout-still-not-solved/