Sorry, should've done this anyway. It's a System.IO Exception:
In German:
{"Der Prozess kann nicht auf die Datei \"C:\\Users\\Admin\\AppData\\Local\\Temp\\AjaxUploaderTemp\\persisted.77d9a535-ddaf-9c3b-a19b-1784e4789435.Unterschrift.tif.resx\" zugreifen, da sie von einem anderen Prozess verwendet wird."}
The English version is "The process cannot access the file because it is being used by another process".
The other process is either the local ASP.NET Development Server or, in a live environment, he IIS Worker Process. The error is thrown no matter if we use Flash or IFrame.
I guess we miss something obvious because not many others have reported this problem so far.
Thanks again.