Failure to communicate with the server:
- it could be configuration issue
- it could be memory issue
- it could be permalinks issue, review this: How to fix 404 error when using Pretty Permalinks? - it could be optimization issue
- it could be a conflict with plugin: try resetting the plugin folder - it could be a conflict with theme: try switching to the Twenty Twelve default theme for a moment via FTP and see if that resolves the issue.
- check for .htaccess file presence and check if the default wordpress mod-rewrite code is there or not. There are times when .htaccess file is not writeable or gotten deleted
- Check the server error log for your hosting account. Ask your hosting provider for assistance to locate them.