Late last Friday night we had a extended power outgage and the systems went down abnormally. Saturday morning everything came up and worked fine. Monday was a normal day. Tuesday morning I was unable to log onto our DEV instance. After shutting down and restarting only one dialog process was working normally in DEV. The trace file for W0 (ST11) looked normal. The trace file for all other process's had a 'call db_connect' at the end. OSS dialed in to our system and eventually recomended that we do a DLTUSRSPC USRSPC(QSYS/QXDADBBK). This is a short description of a very long day.
Sooo, you may want to add this to your list of things to try after system failures. OSS note 93759 describes this problem. In our case we did not have a MCH3601.