I just had a bunch of trouble adding attachments with a match table to another table in an enterprise GEODB. The Add Attachment tool would run for awhile then give an unexpected error, which ESRI suggests is an OS error. I checked and all the file paths were correct. I came back to try again today and got the same error messages and no attachments were added. The results window indicated a different set of 12 or so file paths were a problem. After checking many things including permissions, and discussing the issue with our IT admin.. I finally imported the two tables to a file geodatabase and ran Add Attachments without error. I then Imported the tables to another SQL geodatabase and ran add attachments without error in that database. To fix the issue I deleted the tables from the original SQL geodatabase, then imported them back to that database and was then able to run Add Attachments successfully.
Point is even though the original error indicated an OS problem, it must have been a geodatabase problem! Perhaps related to versioning or to permissions within the geodatabase?