What a clodsburger I am. The problem was had left the old name in Setup.php, (which I need for two wiki on earlier Debian.)

Maybe some warning could be printed ... OK, never mind. Close the bug I suppose, if not closed already.

Here was some stuff I wrote just before discovering the cause:

Yes, changing $wgSitename has instant effect that all users can see. The problem is e.g., $ php dumpBackup.php --current|sed -n '/sitename/{;p;q;}' still shows the old name! (Same with namespace 4 and 5 ($wgMetaNamespace no help.)) update.php doesn't help. Special:Export works fine though. deleteBatch.php and moveBatch.php will only operate on the old name.

Now I have cleaned up Setup.php and all is OK. Maybe some informational messages could be printed...