I figured out why the Slim SEO stopped set pages from working. I am going to write to them and see if they give me a fix, but here is what happens.
On the WP side there are short codes generated by wpForo for the pages. For example the short code is [wpforo] on one of the page.
On the slim SEO if there is no meta description it takes the first copy it finds, and auto ads this to the meta description, in this above case the meta description became [wpforo]. This causes a conflict on the page.
As soon as I insert other meta description without the brackets the page shows and works fine.
It's better to ask slim SEO developers to turn off their plugins on forum pages. They can use the following statement:
if( is_wpforo_page() )
The documentation: https://wpforo.com/docs/wpforo-v2/settings/wpforo-seo/
wpForo has a built-in SEO system with all necessary SEO data in forum pages, you should exclude the SEO plugin from forum pages. The SEO plugins can't work with wpForo. wpForo is not based on custom post types so, your SEO plugin has no chance to work for your forum unless this plugin devs make some special changes to make it compatible with wpForo.
At the Moment only Yoast SEO, All-In-One SEO, The SEO Framework, SEOPress, and Rank Math SEO plugins are compatible with wpForo. These plugins are automatically disabled on wpForo forum pages, allowing wpForo SEO functions to work without any conflict. So saying compatible I mean those plugins don't affect wpForo SEO data and those don't insert any SE information in forum pages. If a SEO plugin is not compatible, it makes lots of problems, it understands all forum pages and topics as a one forum page with the same title and canonical URLs, so Search Engines will see your forum as a one page.