Vbulletin 5 Soft 404 Google Sitemap

I currently have the latest version of vbulletin 5.5.1 in my forum.
On google sitemap am getting Soft 404 error. The pages showing soft 404 error are all accessible via google sitemap or directly and also have content. They are all valid thread or post pages that not deleted, so am wondering why i am getting this error when the pages are valid. Because of this error 500+ pages are been excluded from google index.

This are some of the link google sitemap refered to soft 404.

https://www.trendri.com/forum/business-startup/1200-how-to-startup-cat-fish-farming-business  	Aug 7, 2018
https://www.trendri.com/forum/business-ideas/32-top-five-business-ideas-for-making-money-online	Apr 25, 2018
https://www.trendri.com/forum/finance/209-how-to-apply-for-mortgage	Apr 24, 2018

Also note that this link how not block by robot.txt. How do i get this issue resoved.

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