For a version older than 4.3.6
You can avoid such problem by following the recommendations below:
- Do not upload more than 10 images simultaneously (we recommend to upload 3 - 5 images at a time).
- Use a standard WordPress media uploader (on the same page).
If you still get the same issue open your "wp-config.php" file (located in your WordPress theme root directory on your server) and replace this code
Then check if you get any errors (after upload and publishing).