Some FTP clients, primarily WinSCP, upload a temporary file ending in ".filepart", renaming it to the target name only once the transfer successfully finishes.
Cerberus is getting a file upload notification complete event that is triggering your rule. At around the same time, the client is renaming the file to the correct file name.
Sometimes the file name still exists but the client has opened the file for renaming (this is when you get the "file is in-use" message). Other times the event kicks off right after the file gets renamed by the client. You get an error that the file doesn't exist in this case.
The best way to resolve this issue is to add a condition to your File Transfer Event rule that ignores files with the string ".filepart". For example, you could add a condition that the file does not contain ".filepart".
From there you can handle the rename with a File Move/Copy event where the FROM filename ends with ".filepart". In that case, you know it's the rename after an upload and you can move the file as part of the action for that event.
If possible, the filepart setting can be disabled in WinSCP by going to 'Options' > 'Preferences' > 'Transfer' > 'Endurance' > Select 'Disable' under 'Enable transfer resume/transfer to temporary filename for'... and click 'OK' to save
You can learn move about configuring the Event Manager here.