Unable to add new pages or edit pages etc.

Try a few basic troubleshooting steps:
– Try deactivating ALL plugins temporarily to see if this resolves the problem. If this works, re-activate them individually (one-by-one) to find the problematic plugin(s).
– If that does not resolve the issue, try switching to the unedited default Theme for a moment using the WP dashboard to rule out any theme-specific issue.
– If above steps do not resolve the issue, try resetting the plugins foldervia FTP or SFTP or whatever file management application your host provides. Sometimes, an apparently inactive plugin can still cause problems (because the hooks remain unless plugins completely removed or some plugins stick around in cached files. So by renaming the folder, you break them and force them inactive).

I tried doing all those things.

Still not worked sadly.

– If above steps do not resolve the issue, try resetting the plugins foldervia FTP or SFTP or whatever file management application your host provides. Sometimes, an apparently inactive plugin can still cause problems (because the hooks remain unless plugins completely removed or some plugins stick around in cached files. So by renaming the folder, you break them and force them inactive).
– If all the above steps do not resolve the issue, try MANUALLY re-uploading all files and folders EXCEPT the wp-config.php file and the /wp-content/ directory from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones. Read the Manual Update directions first!
– running the upgrade manually via wp-admin/upgrade.php
– BTW, if you haven’t already done, always backup everything (including your database) before doing any actions, just in case something really goes wrong. It’s a great safety net.