Fighter

Alright found relative_dates_enable on forums_core_sys_conf_settings through phpmyadmin which should be set to 1 instead of 0. Not sure if there's supposed to be something on the admin panel but I couldn't find it. 

This still doesn't work on the newer IPS upgrade from this site... Facebook released a new API in October 2020 and I guess that's the culprit. Facebook and Instagram embeds haven't worked since. 
https://developers.facebook.com/docs/graph-api/changelog/non-versioned-changes/oct-24-2020
 
Found this plugin which fixes it
https://invisioncommunity.com/files/file/9802-instagram-facebook-oembed/

Sorry, the answer was in the topic template itself, since there is a navigation link at the bottom.
The answer is to use "<a href="{$topic->container()->url()}" title="{lang="go_to_forum" sprintf="$topic->container()->metaTitle()"}" rel="parent">" for whatever back button you want to create.
 

Well, I managed to fix it after being really desperate....
Here's the solution in case it happens to anyone else:
if you have access to your server root via ssh, login and delete the tmd file for the table with the problem, in my case I did it like:
rm /var/lib/mysql/*DBNAME*/forums_core_search_index.tmd
Now on phpmyadmin, I selected the table and at the bottom I clicked the repair button and it was fixed without errors. The problem was that phpmyadmin didn't want to overwrite the temporary file.
Now everything seems to be fixed!

Never mind - The way to fix it was to go to
 
System -> Files - > Storage Setting -> Configurations -> Custom URL. The custom URL was pointing to the old folder. Now it is all fixed.