In my case, the command mxpassword -m -x MxDBUserPassword="newpassword" didn't work properly. So I used mxpassaword -g , where you can change the password in a graphical interface.
This can happen when you change the password for the account that the HP SIM services are running under. In addition, if you are using SQL for the back-end (instead of MSDE), you will want to make sure that your SQL account is what you expect it to be. Our system had a service account for the actual services, but it was using the Domain Administrator’s logon for SQL access. This can be changed in "%programfiles%\HP\Systems Insight Manager\Config\database.props". We just changed it to our service account (after granting correct rights on the SQL side of course). If you need to change the password, use:
mxpassword -m –x MxDBUserPassword="newpassword" (without the single quotes) from a command prompt.
From a newsgroup post: "HP Sim uses the local admin account to run the service and connect to the SQL Server's database (on remote server). That was changed to use Local System (as we change passwords quarterly). Resetting it back to local admin worked for us".