Uploading Error: "security token was missing"

[Fynnon]

when i upload small archived files, around <10MB, everything is ok but when i try bigger files i get:

Your submission could not be processed because a security token was missing.

when i uploaded a 5MB file it worked and in the same topic a 20MB file gave me the error
i also tried with the default vb theme but i still get the same error

my vB version is 3.8.6 PL1
CentOs 5.5
Apache 2.2.3
PHP 5.1.6
MySQL 5.0.77

EDIT: changing a line in php.ini resolved the issue

; Maximum size of POST data that PHP will accept.