ImageMagick Path correct but "general error"

When I put the path to ImageMagick and test it (/usr/bin), seems to find it (path correct), but a general error occurs :

Omeka has encountered an error
To learn how to see more detailed information about this error, see the Omeka Codex page on retrieving error messages.

I did both solution to catch an error (log files and .htaccess uncomment "development environment" like the codex page says.
-> no log appears and no other error message.

I've no way to go this problem further. Please help me !

Look here at the Printscreen of the page :

Is there a bug ? What could be de problem ? Am I the only one ? How to have more infos ?