Certainly there is something wrong with your Inbox plugin. Everytime when I activate it, is starts creating other issues.
Yesterday night, I took help from my hosting provider also. They also confirmed that issue is with this plugin only. I deleted that yesterday. Today I again downloaded it from my-account uploaded it, and now site pages are not getting saved. The page keep showing busy and nothing happens. (Updating)
When I tried to deactivate it, firstly it showed me error (<span style=”color: #444444; font-family: -apple-system, BlinkMacSystemFont, ‘Segoe UI’, Roboto, Oxygen-Sans, Ubuntu, Cantarell, ‘Helvetica Neue’, sans-serif; font-size: 14px;”>The link you followed has expired.), the URL was: </span><span style=”color: #444444; font-family: -apple-system, BlinkMacSystemFont, Segoe UI, Roboto, Oxygen-Sans, Ubuntu, Cantarell, Helvetica Neue, sans-serif;”><span style=”font-size: 14px;”>https://ossomrealty.in/wp-admin/plugins.php?action=deactivate&plugin=cubewp-addon-inbox%2Fcubewp-inbox.php&plugin_status=all&paged=1&s&_wpnonce=ad680c8e8e</span></span>
I opened fresh window and tried again deactivating it, then it got deactivated. But pages are not getting saved after saving it.