A: First make sure that your server meets requirements stated here or you can use check file.

Check if you uploaded all files properly - you have to be sure that not a single file is missing. Compare file sizes with sizes within package that you downloaded - some files may be uploaded incompletely. Pay attention to files within /lib/simplepie folder.

Also check if you uploaded hidden files (with special attention to htaccess) - some FTP clients may not have this feature or it is just not turned on.

Compare files and folders names - some FTP clients upload content and turn all characters to small-caps. In that case you should change settings in your FTP client or use another client that will upload content without changing caps.