This thread is resolved. Here is a description of the problem and solution.
Problem: Client has defined the 'OTGS_INSTALLER_SITE_KEY_WPML' in every site's wp-config.php and migrates the site from production to dev every 24 hours.
After updating to WPML 3.9.4, when accessing the backend they get a 500 internal server error.
Solution: There have been issues with the new option in the registration when clients checked the new option, "Keep wpml.org up-to-date about which theme and plugins I use" .
Though the client had not enabled that option, rolling back to version 3.9.3 solved the issue.
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