PHP post max size capped to 20M regardless php.ini settings – Plesk

I have a debian squeeze server with Plesk Panel 11, apache and PHP 5.3.3
I want to increase maximum upload file size but when I try to upload more than 20M, in a single or multiple files, I receive the error:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.

This is my settings:

memory_limit = 128M
max_execution_time = 3600
max_input_time = 3600
upload_max_filesize = 100M
post_max_size = 100M

and also

suhosin.memory_limit=0
suhosin.post.max_value_length=1000000

My apache error log does not contain any relevant reference to the error.

What could cause this 20M cap ?

I tryed a simple page with just a form with one file of 21M in test.php and test.html
The pure html form is submitted successfully. The same HTML code in test.php is not. Hence it must be some problem with PHP and not apache or the server.
I disabled all php extension and removed .htaccess without success.
I also tryed to post data locally from the server command line with curl but I got the same error.

Could be a bug of my PHP version ? Upgrading means to go from squeeze to wheezy, and since it’s a production server I can’t do this so easily