Okay, after some research I found the cause. Disregard the posted code which is a scaled down version of my original code.

My issue was that the uploader was on a panel that was hidded when the page was loaded by use of visible = false

Can't do that... so I used:

pnlPictures.Attributes.Add("style", "display:none")

pnlPictures.Attributes.Add("style", "display:normal")

When showing or hiding panels where the uploader is used.