Hello
I'm getting this error with any email I enter for a sandbox tester :
Unknown Errors while creating Sandbox Tester, Please check Error Log, email=********@*****.****
EDIT: I would also add, as of April 2017 in this SO thread https://stackoverflow.com/a/29280842, that subaddresses work if parent address is used as sandbox too :
if email@email.com is sandbox
email+test@email.com will work
If you try to subaddress your daily main apple id, it won't work.