If you are a Galatea user, contact the administrator of your website.
Open file data/log/error.log and search for a corresponding entry.
Edit the configuration and activate the « Display errors » option in the security tab. Errors will then be displayed on screen.
If you can't access the configuration GUI in the back-end open the file configuration/production/config.php (production is the context used, replace it if differs) and add the following line: