Empty pdf after publish

We have a very strange problem.
Sometimes, but not all the time, when publishing a pdf (publish selection only) the pdf file is empty.

The size of the pdf is exaclty the size of the selection, but there is no content in it.
I've tried al pdf versions, all settings. But nothing helps.

Selecting the design, click on publish pdf. Settings are: (selection only).
Result: Empty file.
Do exacly the same (select again everything I want to publish).
result: Empty file.
Do it again... yes! Score. Now the pdf is ok.

Sometimes I have to do it 5 times to get the pdf out.
Sometimes it works the first time.