Asset PDF viewer shows no content, only icons

I use the assets PDF viewer on synced files for many years without any problems: Users with the link can view and comment the PDFs online. Everybody with the link can see the comments.

Last time, this works without problems, is two or three weeks ago.

Currently I have a big problem:

As you can see in the overview, there is no thumbnail

and in the detail view there are only PDF-icons.

I tried many different ways creating the PDFs and, of cause, the saved configuration I used since many years.

Also the views of old PDF files, which worked perfectly, are currently showing only the icons.

What's wrong?