After trying out multiple suggestions within this thread, I enabled:
define( 'WP_DEBUG', true );
// Enable Debug logging to the /wp-content/debug.log file
define( 'WP_DEBUG_LOG', true );
within wp-config.php.
Next, I repeated uploading an image from within the Media library. No change in behavior yet since I haven’t pin pointed my issue.
Now I downloaded the debug.log file from my SFTP client and spotted the issue right away:
PHP Fatal error: Uncaught Error: Call to undefined function imagewebp() in /var/www/htdocs/wp-content/plugins/images-to-webp/methods/method-gd.php:23
Some poorly written or maintained plugin, I guess. Disabled “Images to WebP” plugin and my image upload worked again.
I encountered the same problem. After some investigation turns out that the function wp_register_plugin_realpath() isn’t specified in /wp-includes/plugin.php.
So I copied the WP 4.0 Beta version of plugin.php from GitHub and replaced it with the current one. Now at least I can reactivate my plugins again. Don’t know of any side effects yet. Please first make a duplicate of your existing plugin.php before updating.
