Invoice PDF Link returns customers without portal to login page instead of the invoice pdf

Describe the bug Invoice PDF Link returns customers without portal to login page instead of the invoice pdf. The customer has NOT an active portal and the corresponding flag in customer administration is turned off. But nevertheless the user is not getting the pdf by clicking the button in the mail. Instead he is forwarded to the login page. :/

Also have unchecked the "expire" function for invoices. Same result. Forwarded to login page.