Problem 500 Internal Server Error

[Caos Unlimited]

Hi everyone, sorry for my english, i know i should improve it.
I tried to upgrate my 1.6.0.9 version to 1.6.1.1 version and the theme lost his style, so I decided to Rollback to 1.6.0.9 but something went wrong and i had "Internal Server Error"
I can't enter in the back office or see the webiste. It just say "Internal Server Error".

Step 1:
- I uploaded my latest back up ( doesn't work - same problem)

Step 2:
- edited config/defines.inc.php  and changed:
PS_MODE_DEV_', false  in to true

 

and @ini_set('display_errors', 'off'  I turned it in 'on'

Doesn't show any error! Doesn't work.

Step 3:
I checked my "error log" file from my admin and it shows:
 

 [error] [client 188.XXX.XX.XXX:XXXXX] -  End of script output before headers: index.php

[error] [client 188.XXX.XX.XXX:XXXXX] -  AH01215: suexec policy violation: see suexec log for more details

 

Now I really don't know what to do. Please, I need help. 
Thank you everyone in advance

 

[Caos Unlimited]

I m Italian, so you can reply in English or Italian language. Thank you again