SharePoint 2013 Owners get Access Denied and HTTP403

On-premise SharePoint 2013.

All of a sudden Owners of sites cannot edit pages, delete permissions, do various administrative tasks without getting an access denied and http 403 error.  We can add permissions, but cannot delete them.  Cannot edit pages or check in pages.  You can checkout a page, but not check it back in!  Anyone had this issue?

Just some extra info which may help you - Every now and then we find that our database for this particular web app goes into single user mode in SQL!  Not sure why this happens.  Occurs at night when we do backups.   I am also trying to get Dynamics CRM and SharePoint integrated with a different Web App and with difficulty!  Not sure if I have broken something to this particular web app!  I also had to recently fix up the MetadataServiceProxy which was inaccessible and is now happy.

I can send ULS logs if required - too long to fit here.  Please let me know where to send to.

Thanks in advance!

Kind Regards.