Some images can be loaded through customer.fcgi and some does not

Hi fellas,

Im looking for help from you old-school onsite folks with an extremy odd issue.

An onsite (belive me, we are pushing for them to migrate..) have the issue that neighter them, or their customer can see some images using customer.fcgi, but, using rms.fcgi as an logged in agent works fine.
It also seems very random, in the same ticket, some images works, and some does not.
For example the logo from the customer works in one message, the exact same logo doesn´t in the next.
Fetching the base64 from Attachment works just fine and is the correct image, as well as renaming the file from /attachments

Scenario: 
An external ticket, the message is external.
Running:
http*://XXXXXX/service/scripts/customer.fcgi/getAttachment/2133444-c9lqX8IQjZNj7p8J0EMhq2g4xdhK8R9opUiaH7L8Sp0GMOeGTPd9njEpWyBJhiJP-0/image002.jpg
returns a 401
But, running http*://XXXXXX/service/scripts/rms.fcgi/getAttachment/2133444-c9lqX8IQjZNj7p8J0EMhq2g4xdhK8R9opUiaH7L8Sp0GMOeGTPd9njEpWyBJhiJP-0/image002.jpg works just fine.

SuperOffice: 10.2.5
Pretty standard setup, SSO, Sales in the same site, no split attachments, old customer from around ~v.6.x

Clues: The customer told me that they also have noticed that an image can work just fine, but, after some mails back and forth to the customer, the previous image(s) starts behaving this way.

Can be reproduced in test enviroment.

Database entries:
Attachment_path: C:\SuperOffice\Service\attachments
cgi_bin: /Service/scripts
cgi_url: https://....se
cgi_url_internal: https://....se
Path or URL to www folder: /Service/

Big bag of swedish candy for anyone that have an idea, im loosing my mind here :)

Edit: tried turning of SSO, users are promted with login, but, still customer.fcgi does throw a 401

Regards
Pär Pettersson