Upgrade to 4.0 almost all images gone

Hi,
i have a blog on a natural reserve served by WordPress https://www.urseetal.net. When I updated to 4.0, the largest part of images, but not all, is no longer avaible in the media catalog neither in the old list, nor in the grid view.

This is neither dependent on any plugin, I deactivated them all, nor
on the theme which is Emptiness or Twenty fourteen. The images are still in the upload folders but the program has lost during the update the information about their size. These are both jpg or png images.

What to do? This problem has been asked in the German forum already with no solution.

Bernhard