TYPO3 Core

After updating from 4.2.10 to 4.2.12, the web modules ("Page" to "Template") are not loading any more.
Instead of displaying the frameset with the pagetree left and the edit frame on the right, there is a blank screen (well, the modules frame on the far left stays in place of course).

The Firebug network monitor says (for a "Page" module request):

GET http://www.ur-sprung.at/typo3/alt_mod_frameset.php?fW=0&nav=/typo3/alt_db_navframe.php%3F&script=sysext%2Fcms%2Flayout%2Fdb_layout.php&id=

Response: [nothing], Content-Length: 0, but HTTP 200 OK

The Firebug error console states no JavaScript errors.

This was tested with FF 3.6, Opera 10.10, Chrome 4.0 and IE 8 - result is all the same. I am logged in with admin rights.
(issue imported from #M13639)