I've solved with 2 steps:
- using firebug in FF i've seen that the config sections added in web.config made a wrong url:
http://crystalreportviewers13//js/crviewer/crv.js
If i remove these config sections the url is well formed
The last step is doubling the report folder because as default the report is deployed under bin folder but in the code i read a report folder in web app root.