In order to change the password for the 'admin' user in Momentum 4.1, you will need to execute a NodeJS script from the Platform node where the Web UI is hosted. You will need to execute the following commands (be sure to remove the comments preceding the commands):
The script below relies on a relative path, so be sure to execute the script from within the above directory:
/opt/msys/3rdParty/bin/node etc/update_user.js --NODE_ENV=production --username admin --password password --customer 1
Once this is complete, you should have no trouble logging into the Web UI.