I've helped two people in
#wordpress this morning with this same error message, so I thought I'd pass the info along.
If you get this error:
Fatal error: Call to undefined function: wp_cache_flush()
You probably need to re-upload your WordPress files. Make sure the PHP files are uploaded in ASCII mode (consult your FTP software's help files to see how to set this). You also should consider deleting the
/wp-includes/ directories and replacing them altogether. There should not be any customized files in these directories unless you edited them yourself. Sometimes FTP uploads on top of existing files can have strange results.