Hallo Claus,
ich denke hier ist die Erklärung:
Using All Numeric Passwords with iSeries Access for Windows (Client Access Express)
Technote

When using all numeric passwords, the initial logon for iSeires Access for Windows does not require you to type the preceding Q when your system value QPWDLVL is set to 0 or 1. This also applies to changing your iSeries ™ password through the iSeries Access for Windows interface. iSeries Access for Windows will automatically append the Q when it sees an all numeric password.

If the QPWDLVL system value is set to 2 or 3, the password is not altered in any way when sent through by iSeries Access for Windows. Therefore, the preceding Q must be typed by the user. This is also the case when you change and iSeries password through the iSeries Access for Windows interface.