This error occurs because the file you are trying to upload is open on your computer. In order to fix this be sure the file you are trying to upload is saved to your computer and closed(cannot be open for editing or viewing), then close out and re-open PREO. Restarting PREO is necessary in order to clear the error. You should then be able to upload as normal.
Uploading file failed: 400: Unable to parse form content
How to resolve error when uploading documents to PREO.
Written by Kyle Atkinson
Updated this week