Discovered that the IT department was running a scheduled "clean" script of the temp folders and cache for random users. Which removed the files from the user's temp folder and cache. User was unable to access file.
Had user cancel checkout through Windchill. Checked the file out again and make changes and create new iteration.
Also, have since had IT remove the script so that it does not run on Windchill users.
Seems to have resolved the error.