Hi again, krileon, and thanks for answering so quickly.
I have tried it from my Tablet, and it also shows that screen. But again I tried from my desktop comp, and it worked. Then, I disconnect, and I tried to connect again, and it failed.
I have switched off all the cache plugins and the Joomla cache conf option. Anyway, it shows the blank window.
However, I have realised you may be right, because each time I clear the cache, it doesn´t fail, but fail the following time.
What can I do?