some Users can't login to SPO

The users are all member of the security group 'S_contoso' which are in the site default SP group 'visitors'

'Everyone except external users' are also in the Visitor group

when a user logs in - they briefly see the site default page, but gets redirected to the 'access denied' page and they can request access..

The frontpage have some doc library webparts - and each doc library have special permissions - not inheriting permission from site 

• If I approve the user request - they still can't log in  
• giving them full control (owners) - doesn't work either
• It doesn't help empty the IE cache 

but if I promote the user to site collection admin they get in and are not redirected to the request access page - then removing them afterwards from site collection admin - they still gets in ..

anyone knows whats wrong or missing?