[Resolved] 404 error on opening pages

Background of the issue:
I am trying to open pages one by one on my website, but after opening a couple, it gives a 404 error. The issue can be seen on this page: hidden link

Symptoms:
I expected to see pages without the 404 error. Instead, I got a 404 error.

Questions:
Why am I encountering a 404 error when opening multiple pages?
How can I resolve the 404 error on my website pages?....