If you change the db2admin password at the Linux prompt your simdbmu.log will show User Name or password errors.
Some user credentials exist in 2 places, both in Linux and in LDAP. If you change one your SSIM will have a mismatch and will not work properly. The simdbmu service will not be running.
You must always use the SSIM Web Configuration page to change system passwords such as db2admin.
The cn=root password is changed by following the steps in this document.
Users that are created through the SSIM client are changed within the SSIM client.
If you have already changed the password at the prompt and are now having issues, try to use the SSIM web configuration page to change the password again to the new password, meaning all three fields in the Change Password screen will be the same.