The issue possibly started on Sept 30 when the domain connected with the WordPress instance on AWS expired.
As the site went down because of domain expiry, I connected with another of my domain with the aim of moving hosting from AWS Lightsail to WP Engine.
It is only after migration to WP Engine it came to my notice that forum pages (using bbPress plugin) not opening.
For instance this forum topic page: https://progannum.com/?topic=comparing-list-manipulation-in-wolfram-language-vs-python
So checked the copy on AWS Lightsail as well and there too forum pages not opening. I can still access the content via edit mode. So the content is not lost, either on WP Engine copy here (progannum.com) as well AWS Lightsail (digitalsplendid.xyz).
Someone suggested to refresh by clicking save on permalinks, which I did on progannum.com. After that getting a complete blank screen instead of earlier 'This page doesn't seem to exist' as still visible on AWS Lightsail WordPress copy here: https://digitalsplendid.xyz/forums/topic/comparing-list-manipulation-in-wolfram-language-vs-python/.
Got this suggestion to refresh permalink from my earlier Reddit post which I did: https://www.reddit.com/r/Wordpress/comments/1fv9c3c/comment/lq5i742/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button
UPDATE: The WP Engine chat support helped me this way:
Also provided this thread for reference: https://wordpress.org/support/topic/blank-topic-pages-on-bbpress-while-using-twenty-twenty-two-theme/
Since the domain name has changed, you need to perform a find/replace operation on the DB, as WP generally stores URL as absolutes – that would likely be why you’re getting 404s. It also helps to check DevTools > Network tab for errors.
This can be done via WP CLI search-replace or a plugin called Better Search Replace.