uploading pic(s) – selection field disappears

hey there,

I’ve got a problem. At a moment’s notice the selection field to choose files in, disappears. I really don’t know what happened ’cause the rest of my wp installation runs fine. When I press the link to insert a pic, the box shows up and I can see the field for a second but no longer. It disappears and only a button “choose your files here” is left.

Any ideas how to fix it?

thx!