Which config file are you modifying?
How are you launching amuleweb? Are you launching it manually, or letting aMule (or amuled) launch it automatically?
If you launch amuleweb manually, then it uses ~/.aMule/remote.conf for its configuration file, and that determines the password it requires for login. If you let aMule or amuled launch it automatically, then it uses ~/.aMule/amule.conf for its configuation.
If you do not have a remote.conf file, you can have amuleweb create one based off of amule.conf with the following command:
amuleweb --create-config-from=~/.aMule/amule.conf
That only needs to be done once to create the initial remote.conf.