Upload error 500

Can anyone tell me what is Upload error 500?
I am getting it upon a completion of mp3 file upload.
Smaller files (8mb) upload ok but 20 mb file gets 500 error.
I use Joomla 2.5.1
Here is what in my php.ini file:
rg_emulation=off
memory_limit = 500M
post_max_size = 100M
file_uploads = On
upload_max_filesize = 192M
max_execution_time = 300
cgi.fix_pathinfo=1

Thank,
Igor

Sorry, I've somehow missed your post.
In case you didn't figure it out already or someone else finds this thread:

This error usually means that the server encountered an internal error. Given the fact that larger files give you this error, it is most likely caused by the size. Since you already raised the php.ini limits, you should now check the server error log to see what the problem is.