Hi all,
Finally I found the reason of this problem yesterday.
In my case, the problem occurs when the path containing files I want to join, has a "Ñ" (spanish character) as part of any folder. It happens with some other non international characters too. It's difficult to identify the problem, because you don't receive any error code: only a blank page document is generated without giving an error.
Thank you for the support of PDF24's team, who has tried to help me asking for debug logs and samples.
It's a very very usefull tool, I will keep on using.
Juan I.