Thank you greg for your interest (and thank you Chris! just seen your post..)

I'm on a local environment WIn7 64bit + IIS7.5 + SQL Server 2012 r2

Since today log file is empty and in the event viewer there is no signal of exceptions (except the 404 ones), I attach here yesterday logs: http://www.hakkar.it/logs.zip

Also I found this topic which describes a very similar issue:
http://www.dnnsoftware.com/forums/forumid/107/threadid/490105/scope/posts 

I don't understand the soluton proposed. Why should I add that attribute to modules section thus triggering all httpmodules for every request (even jpg css or js) and stressing system resources more.

Here is the 404 exceptions from today:

 

Don't know if it's realated but just noticed my login popup is empty as well... register popup just works fine.

I'm sorry to spam.. if I could edit my previous posts I would post updated in a single post. Anyway... I managed to fix the login popup issue this way:
I logged adding to the site domain this: www.mysite.com/default.aspx?ctl=login (thanks to http://www.dnncreative.com/Forum/tabi... )
I added the login module to another page
I went in admin > site setting and set that page for login (checked and it worked)
I went again in admin > site setting and removed the custom page for login

At that point the pop-login started working again... I'm still undecided if roll back to 7.2.2 or not since sites are empty it was an upgrade from almost empty site in a development environment. That's why I thought "well let's start with the latest version" but it seems rather buggy at this stage yet...