I've got an external user who has had access to a SharePoint 2016 site before. He couldn't get in today and keeps getting the 'Access required' no matter how many times i send him a Sharing Request add him to the groups or Approve his requests he keeps coming back to the "Access Required" page. Any ideas?
You could go to user inormation list, delete the external user there. Then add the user in the groups again.
The url of user information list: https://yoursites/_layouts/15/people.aspx?MembershipGroupId=0