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Since you downgraded, you probably didn't follow the full instructions. My bet is that you're sitting on the modified 3.41 that was used for downgrading from 3.55, which technically has the firmware version of 3.5_6. In order to be able to upgrade, you'll need to downgrade your PS3 to a firmware that is lower then 3.41 and greater then or equal to the firmware that came with your PS3 (3.40, 3.20, 3.15, etc) and then once on that firmware you should be able to upgrade right to 3.55.
Thank you very much.. I think you are right.
But can i downgrade with 3.41 to 3.15, and were can i download the modified version of 3.15, ore can i just install the normal 3.15? :P