I doubt this has anything to do with your admin user.
You say the host name has been changed. Likely you have a <site>
definition for your site that sits in a wrong place in the order of sites that serve up Sitecore.
<sites>
<site name="shell" virtualFolder="/sitecore/shell" physicalFolder="/sitecore/shell" rootPath="/sitecore/content" startItem="/home" language="en" database="core" domain="sitecore" loginPage="/sitecore/login" content="master" contentStartItem="/Home" enableWorkflow="true" enableTracking="false" analyticsDefinitions="content" xmlControlPage="/sitecore/shell/default.aspx" browserTitle="Sitecore" htmlCacheSize="10MB" registryCacheSize="15MB" viewStateCacheSize="1MB" xslCacheSize="25MB" disableBrowserCaching="true" />
<site name="login" virtualFolder="/sitecore/login" physicalFolder="/sitecore/login" enableTracking="false" database="core" domain="sitecore" disableXmlControls="true" />
<site name="admin" virtualFolder="/sitecore/admin" physicalFolder="/sitecore/admin" enableTracking="false" enableWorkflow="true" domain="sitecore" loginPage="/sitecore/admin/login.aspx" />
<site name="service" enableTracking="true" virtualFolder="/sitecore/service" physicalFolder="/sitecore/service" />
<site name="modules_shell" virtualFolder="/sitecore modules/shell" physicalFolder="/sitecore modules/shell" rootPath="/sitecore/content" startItem="/home" language="en" database="core" domain="sitecore" content="master" enableTracking="false" enableWorkflow="true" />
<site name="modules_website" virtualFolder="/sitecore modules/web" physicalFolder="/sitecore modules/web" rootPath="/sitecore/content" startItem="/home" language="en" database="web" domain="extranet" allowDebug="true" cacheHtml="true" />
<site name="website" enableTracking="true" virtualFolder="/" physicalFolder="/" rootPath="/sitecore/content" startItem="/home" database="web" domain="extranet" allowDebug="true" cacheHtml="true" htmlCacheSize="50MB" registryCacheSize="0" viewStateCacheSize="0" xslCacheSize="25MB" filteredItemsCacheSize="10MB" enablePreview="true" enableWebEdit="true" enableDebugger="true" disableClientData="false" cacheRenderingParameters="true" renderingParametersCacheSize="10MB" />
<site name="scheduler" enableTracking="false" domain="sitecore" />
<site name="system" enableTracking="false" domain="sitecore" />
<site name="publisher" domain="sitecore" enableTracking="false" enableWorkflow="true" />
</sites>
You need to make sure, your custom site definition (the one you just changed the host name for) comes after the Sitecore sites ("shell", "login", "admin", "service", "modules_shell" and "modules_website"). Otherwise your site will try to process the login request and ultimately fail.
How have you changed your site definitions after the working vanilla install? Normal procedure is to patch in new sites just before the "website" site definition.
<site name="custom" patch:before="site[@name='website']"
hostName="customhost"
virtualFolder="/"
physicalFolder="/"
rootPath="/sitecore/content/custom"
startItem="/home"
database="web"
domain="extranet"
allowDebug="true"
cacheHtml="true"
htmlCacheSize="50MB"
enablePreview="true"
enableWebEdit="true"
enableDebugger="true"
disableClientData="false"/>