Hi all,
it is now working again. I am however not exactly sure what did finally the trick.
I deleted the cache of my browser (IE) but this did not work in the first place.
It did also not work in other browsers (Chrome)
I also rebooted my notebook and again cleared the cache.
Then it worked.
Still I find this a strange behaviour for a simple functionality as uploading a picture.
Thanks for all the hints and ansers.
Best Regards,
Andre