Ok, I found out that the problem with the blank page was caused by the "recent comments" block in the front page.
The blank page appeared to guests (logged out users) so it was difficult to catch it
because I've the automatic log in turned on. So it appeared during the chat session and I disabled all the blocks and turned them on one by one and got it.
I don't know *exactly* why it happens, I built that block from scratch by myself (that's probably *why* it happened
) and I'll try to make it work as it is supposed to work, meanwhile, I disabled it for guests, one can see it only when it's logged in, that should solve the problem.
In any case, if you see again the blank page, be known that you can reach the site using, for example, the forum page...
Thank you for having warned me