Hi Ken,
Thank you for your reply.
No, a test page like that does not get the same problem.
Some more background information about my situation:
The CuteWebUI:UploadPersistedFile is part of a custom user control for uploading and resizing images, which is added to the page dynamically in the CreateFieldSet method of a subclass of DetailsView.
But when used in the DetailsView context, so dynamically added in CreateFieldSet , it's getting the error I started this forum thread about.
But... I do not always get that error. Sometimes it works just fine. I can't put my finger on it... that's why I'm asking for help.
Regards, Marja