It's definitely dovecot and the path to the mail_location.
If I change it to:
mail_location = maildir:~/Maildir
the normal user accounts work fine but the admin accounts don't work.
Error with admin is "No such file or directory" when trying to create the cur under Maildir. The admin accounts have a Maildir directory under home but when I ssh in as an admin user I can't cd into it. I'll continue to work on trying to fathom that bit out but if anyone has any ideas let me know!