OpenID Provider [REGARDED AS SOLVED]
The documentation says "They also can work as OpenID server, but only if the Wiki adminstrator has enabled this feature", but I cannot find how to enable it in the more detailed documentation. My fault?
Solution: in your LocalSettings.php set
$wgOpenIDClientOnly = false; // false means: allow to act not only as client, but also as OpenID server (you wanted this) $wgOpenIDAllowServingOpenIDUserAccounts = true; // check, if you want this. For tests, set this to true
The article page and the README explain the meaning of all switches.