Sorry, I should have mentioned. I did check logs at the time of the error and there were no errors recorded in either ColdFusion or IIS logs.
THe chain of events is as follows:
I add a file and click Upload.
The file's progress bar turns green and shows 99%
The onComplete event fires and shows an alert. I close the alert
The file progress bar immediately turns red and says Error.
Overall progress bar is green but still says "Uploading 1 of 1" (or 2 or whatever).
No other files are processed, but the first file IS on the server in the appropriate place.
Now I am thinking there must be something else that CF is trying to access that it does not have permissions to. I run CF under a local machine account. I've given it full permissions to the Coldfusion folder and all subfolders. I am not familiar with Flash. Is there something else it could need access to?
Okay, well I got it to work. I don't really understand the logic, but when I added the "results" parameter to the cffile upload all tag, the error went away.
Just in case anyone else has this problem, but here is my tag from the upload page:
<cffile action="uploadall" filefield="filedata" destination="#uncDirectory##url.folder#" nameconflict="makeunique" result="uploadResult">