Roundcube webmail and DOMDocument.php

[f4b10]

Hi, i've an error in my log when i try to read mail with HTML switch to on, in my log appears this...

[19-Mar-2010 22:11:58] PHP Warning:  include(DOMDocument.php) [<a href='function.include'>function.include</a>]: failed to open stream: Nessun file o directory in /home/admispconfig/ispconfig/web/roundcubemail/program/include/iniset.php on line 99
[19-Mar-2010 22:11:58] PHP Warning:  include() [<a href='function.include'>function.include</a>]: Failed opening 'DOMDocument.php' for inclusion (include_path='/home/admispconfig/ispconfig/web/roundcubemail/:/home/admispconfig/ispconfig/web/roundcubemail/program:/home/admispconfig/ispconfig/web/roundcubemail/program/lib:/home/admispconfig/ispconfig/web/roundcubemail/program/include:.:') in /home/admispconfig/ispconfig/web/roundcubemail/program/include/iniset.php on line 99
[19-Mar-2010 22:11:58] PHP Fatal error:  Class 'DOMDocument' not found in /home/admispconfig/ispconfig/web/roundcubemail/program/lib/washtml.php on line 247

i've try to find it, with

locate DOMDocument.php

but this file can't exists...

i've already installed libxml support and in my phpinfo(); are all enabled..

how can resolve it?
thanks!