We found the root cause of the problem: the domain time was out of sync. I synchronized the domain controllers’ time clocks with the PDC. After the rest of the network resynced, Epicor attachments worked again.
If this happens to you, make sure you check the time difference between your DocStar server and your PC. Be aware of Hyper-V trying to overwrite your DC-set time clock. I believe that was the root of the issue.