Thanks for updating the thread.
I see that you have child theme installed. Please try and activate parent theme from Appearance > Themes and see if doing so resolves the 500 error message for blog posts. If it does then some customization is effecting the same which you need to take a look.
If the issue is still there, please share the website login details in secure note for for us to take a closer look. To avoid confusion and slow response time, please create a new ticket as the 500 error message is a new issue that's showing on the website and not quite related with your first question. So to avoid long threads, confusion and slow response time, please create a new ticket and give reference to this ticket and share website login details in secure note for us to take a closer look.