You have a server side issue, your server's max data size that can be sent to your server is 50M, max file size that can be uploaded to your server is 50M, and max number of files that can be uploaded via a single request is 20. Some users are trying to upload files more than 64.39 M to your server. To fix this issue please contact your server provider and increase post_max_size, upload_max_filesize and max_file_uploads.