Well there's a good chance that it is the oldest file in your Aux 1 folder that is causing the error, whatever the error is. You might try deleting the oldest file manually. If it works it works, if it doesn't, then maybe you get more detail about why.
File deletions can fail for a number of reasons:
1) Disk hardware error.
2) The user account attempting deletion doesn't have permission to delete.
3) The file could be flagged read-only (this actually is enough to prevent some delete commands from working).
4) Some program has the file open.