more than likely your host has PHP memory_limit set to 8Mb. Tiki needs more, See the Home page for a link to the PHP memory_limit article I wrote about this problem. Also see TikiFriendlyHosts for hosting providers which can run TikiWiki without problem.
Register at tiki.org and you'll be able to use the account at any *.tiki.org site, thanks to the InterTiki feature. A valid email address is required to receive site notifications and occasional newsletters. You can opt out of these items at any time.