Could not open the log file “/dev/stderr”

For this plugin to work, PHP needs to be configured to log errors to a regular, readable file. It looks like on your server PHP is set to output errors to stderr, which is not a physical file but an output stream that could be shown in the console, redirected to a different file, etc. In any case, the plugin can’t read data from stderr.

Try to create an empty log file somewhere on the server (preferably in a location that’s not accessible to visitors) and then configure PHP to log to that file. You could do that by editing the PHP configuration in your php.ini file, or maybe by adding a line like this to your wp-config.php:

ini_set('error_log', '/full/path/to/log-file');