After uploading a particular pdf file (of size 4MB) the message
"Error creating thumbnail:
- Warning: File has insufficient data for an image."
appears, where the "warning" line is repeated 5 times. The message closes with "convert: missing an image filename `[...]/images/tmp/transform_[...]-1.jpg'."
However, if I click on "Full resolution", the pdf file can be displayed by firefox/acroread. But there is no thumbnail/preview.
What could be the reason for this? (What do I have to do to isolate the problem?)
Other pdf files don't make such problems.