[TYPO3] Contextual menu in Typo3 4.0 doesn't work

[Tiziano Fogliata]

Hi,
today I've discovered that contextual menu in Typo3 4.0 backend doesn't
work.
It worked since yesterday but now I cannot use it at all.
The JavaScript console in Mozilla Firefox reports these error

Source file:
http://www.xxx.com/typo3/alt_clickmenu.php?table=pages&uid=1&listFr=0&enDisItems=&backPath=|6521843081&addParams=&amp;bank=0&ajax=1<http://www.xxx.com/typo3/alt_clickmenu.php?table=pages&uid=1&listFr=0&enDisItems=&backPath=%7C6521843081&addParams=&amp;bank=0&ajax=1>
Line: 1, Column: 2
Source code:
 <?xml version="1.0"?>-

and

Error: t3ajax has no properties
Source file:
http://www.xxx.com/typo3/alt_db_navframe.php??currentSubScript=db_list.php
Line: 141

May somebody help me?

Thanks.

Best Regards,
Tiziano.