The first error message on screen may be a bit misleading as it mentions "authentication" - however that is just a side effect of the actual underlying problem. The messages in the STDlogs show that that the system is not able to find the physical file which the attachment record is pointing to.
There are two common causes for this problem:
1. The system with the problem was created as a copy of another environment - such as a Test or Dev system that was created with Data and Customizations from Prod. However, the attachment files were not brought over to the new (copied) environment, and thus the attachment db records point to files that do not exist on the server.
2. The repository location was changed on the repository record itself, however the physical files were not moved to the new location, thus the repository is looking for a file in the wrong place.