I was going to create a few draft posts for keeping the idea, but then after doing for three of them, I figured out that all new posts had the same post_id, thus it was like I revised the same post on and on.
When I created draft posts, I just right-clicked ‘Add New’ button on top of /edit.php page for three times. Usually, before my server migration, I ended up with three blank posts, and adding titles and contents, automatically WordPress assigned a new post_id to each post.
Now it seems that WordPress does not recognize the fact that I created a draft in another window in the same browser.
What’s more problematic is that I have to wait very long time to have another post_id assigned to new post. Till then, whenever I click ‘Add New’, I end up with the same post_id, and repeating the ‘unknown revision’ all over again.
I thought it is due to any level of caching, so I purged existing cache, deactivated caching plugins, and changed server settings in nginx to deactivate browser caching or so.
None of them are effective.
I doubt it is due to WordPress’s native error, but still, I can’t pin point the root cause of this strange situation.