SharePoint 2016 all users can access all documents and folders but getting "Access required Sorry, this site hasn't been shared with you" when adding an event to the calendar (Web Part) and the 2 users that have contribute and edit were able to add 4 days ago.
Please check if the calendar list has unique permission.
Go to calendar list> list settings> Permissions for this list> make sure you have the right permissions.
If the issue still exists, browse to the site and edit the URL by adding the following string to the end of it:
For example, the full URL will resemble the following:
2)Select the person from the list, and then on the Actions menu, select Delete Users from Site Collection.
3)Grant the user permissions back to the list.