Answering to myself
It was due to plugin "Yet another stars rating"
To know how to solve this:
- I entered the FTP
- I renamed the plugins folder as 'plugins2'
- I went to this url : http://www.abcargent.com/wp-admin/plugins.php - this should disable all plugins and you should not see any plugin available
- I renamed back the folder to 'plugins'
- I entered again to the same url.
- Now all plugins are here again but disabled
- I enabled each plugin one by one
And finally found the one that would make everything fail. Once i click enable on this one, i get again the 500 error.
Hope this can help.