Try with a standard skin.

Your skin probably has a custom comment form that does not pass the security crumb. This crumb is necessary for security reasons.

What do you mean by a Standard skin?
I'm using the Aventura skin from b2evolution Skins and I get that message sometimes, not all the time. Unfortunately I haven't found out which circumstances generate the message.

@famimama: You need to have cookies enabled for this to work properly

@gcasanova: I've seen this more often in the forums. The crumb is session related and glitches like it works sometimes most often are. If you want to dig into this you'd have to debug function /inc/sessions/model/_session.class.php

function assert_received_crumb( $crumb_name )

on line 549
You can debug by adding

pre_dump( $Session );

or

global $Debuglog;
$Debuglog->add( "Crumb stored : " . $crumb_recalled, 'crumb' );

For the latter set $debug to 1 in /conf/advanced.php

The above are only guidlines. You'd need to tinker with the variables.

Good luck