could be many things, but check those two:
1. Check write/modify rights for the User Account that's being used for the Execution Service, for the specific folder.
2. Check if you have any "ghost" processes that are left over from previous crashes. Those left overs can be "holding" a file preventing it from being re-written.
3. See if you can get any clues from the log file - what's the recorded error that's causing the failure?