Thanks a lot. I have some news...
Usually, I use Firebug in order to spped up debug and developement tasks but his console still not display any error or any useful info when I load the event creation page.
But, in the meantime, I found the solution!
For your information:
My hosting provider is SiteGround and they developed a plugin for Joomla that allows some cache improvements in order to speed up the site. I tried that plugin but it was not satisfy me. I decided to use Google Page Speed instead.
Then, I disabled the dynamic cache from SiteGround cPanel but I forgot to disable the related plugin in my Joomla environment.
Now I disabled the plugin and all is working correctly!
In the attached image you can see a schreenshot of the related page in the SiteGround cPanel (sorry but it's in italian).
I think to not use that plugin in future but If you want to analyze this conflict I can send you the plugin package.
Thanks a lot for your quick answer and great support! (and sorry about my poor english...)