Foren / Cotonti / Support / [solved] Strange urls after update

The problem is that ajax worked fine in Siena 9.8 before the upgrade, after I upgraded the site to Siena 9.11, it stopped working. :/

When I reload the page with hashed url, it blinks for a while (I can see a loading animation) and then I get a blank page. 

base.js loads fine.
htaccess and settings are set on built-in Cotonti pretty urls.

Added 4 hours later:

Strange thing: when i copied the same site to localhost, the ajax works. 

I noticed that my hosting provider attaches some kind of javascript to the pages. Can this cause errors? But previously, ajax worked fine... Hmmmm.