I think the password can not be changed after editing the XML-config. I think there is a bug in the firmware, because the name of the admin is also not changeable via the WEB-GUI because the name is hardcoded in the XML. So when you change the name of the admin in the XML-Config, the WEB-GUI "can't find" the user whose password should be changed.
Try to rename the admin-name to default, change the password via WEB-GUI and change it back to your custom name. This is an awfull workaround but as long as the name of the admin-name can't be changed via WEB-GUI, I think this is the only way.
If you've got a Service-Contract on your AP541N make a service-call an request this "feature". If cisco got many of this requests, perhaps they will fix this issue in an upcomming release.
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A Wireless Access Point (WAP) connects to a router and serves as a node
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