We have since the update to 7.16LTS version a new strange behaviour.
Sometimes, not reproducable till now, we get the message "unable to display format", even if the format is pdf.
For *.zip-files it's ok and understandable but for all the others?
We can reload the page (e.g. F5) an with reloading the message dissapears and the preview is correctly.
How can we fix that?