in Qlik Sense the section access table doesn't have NTNAME field anymore but only USERID field.
USERID for domain users must be as "domain\username" of course.
Have your tried to check if the domain name registered on licenses have a prefix instead of a single user name?
Anyway, the Section Access must be applyed on the APP, the document reloading and publishing it in a stream. All fields must be uppercase also.
Recommend that you close the Browser then try to access in the stream.
The full and correct that you want to test is this:
LOAD * inline [
ACCESS, USERID,REDUCTION, OMIT
NUM AS REDUCTION;
Chr( RecNo()+ord('A')-1) AS ALPHA,
RecNo() AS NUM
Take a look, the DOMAIN or COMPUTER name must be set as the same on license allocated for both users.
Do you know if this problem isn´t happening without a Section Access? Have you checked if this users have token allocated?