Some versions of the GD library don’t play well with transparent images (PNGs). Maybe this is the case? I’d recommend switching your hosting provider, 1&1 is very … let’s say … special.
You are right! 1&1 is very special. I found the issue.
The images that i want to make smaller with the thumb function are to big. A image has around 4-5 MB. I need this because this are headimages that fill the whole display in the width.
For a test i resize the image manually and now the thumb function works.