I'm sorry for having missed the obvious: my updatedb was not running correctly, so locate didn't really work. It seems that on AWS most of the logs are in /sap/usr/HDB/HDB00/imdbhdb/trace and the important log file is indexserver_alert_imdbhdb.trc. On our AWS instance this file was cluttered with messages of the type
[4709]{423385}[47/1176950] 2013-11-21 13:47:01.249781 e TransactionManag TransTokenManager.cpp(00497) : TransTokenManager::getPartitionID: a negative CPU socket index is returned - fallback: a dummy partition ID is assigned
and so the really important messages were not really visible (see http://scn.sap.com/thread/3409932)
-- Micha