Hi Lukasz,
I identified the problem (I verified it also with 3.5.2).
This is exactly the bug I'm experiencing: http://www.ez.no/community/bugs/kernel_20_error_on_image_upload
But, on 3.5.2 you obtain the error above, on 3.6.0rc1, I see the message I wrote in the first message.
Could it be some sort of Apache 2.0 misconfiguration?
Hard to say without reproduceing your error. I`m using Apache 2.0.54, PHP 4.3.11 and MySQL 4.1.12 and works fine with eZ publish 3.6.0rc1 and 3.5.2 (Virtual Host installations)
