[xwiki-users] XWiki 7.4 Distribution Wizard says: "We couldn't find any extension with id 'org.xwiki.enterprise:xwiki-enterprise-ui-mainwiki' and version '7.4'."

Hi, when working through the Distrbution Wizard of our freshly installed XWiki Enterprise 7.4 (WEB installation), at step 1 (User Interface) we are faced with the following message: " We couldn't find any extension with id 'org.xwiki.enterprise:xwiki-enterprise-ui-mainwiki' and version '7.4'. Make sure you have the right extension repositories configured. " The complete creenshot can be found at http://imgur.com/Uc2F9pr. I've copied the file "org.xwiki.enterprise:xwiki-enterprise-ui-mainwiki-7.4.xar" (from http://extensions.xwiki.org/xwiki/rest/repository/extensions/org.xwiki.ente…) into WEB-INF/extensions to no avail. The installation instructions doesn't mention any required step regarding the configuration of extensions or extension respositories or where these respositories are. Therefore I'm a little bit at a loss here. Our system is running on OpenSuSE 13.1 with PostgreSQL 9.2.14 (from repo), Apache Tomcat 8.0.30 (manually installed) and XWiki Enterprise 7.4 (WEB, manually installed). Any pointer to fix this problem are appreciated. Cheers Frank