That could be due to WAS Plugin sends a different encoding than the incoming request is used. By default, the WAS Plugin uses the URL that has been decoded by Apache and split into various sub-components. It can then re-encode the components in unexpected ways. In this case, the get user URL request was decoded in an unexpected way after it route through the web plugin, hence why no users are returned.  Workaround is to configure the web plugin to start with the URl the way the client encoded it and do no further decoding or encoding. The apache environment variable websphere-nocanon turns this feature on. Below are the detail steps to turn on this feature:

Navigate to the installation directory for your IBM HTTP Server.
Open conf\httpd.conf in an editor.
In the Global section (Section 1) of the file, add this entry:
SetEnv websphere-nocanon 1

The following site describes the details of the above plugin setting:

: http://publib.boulder.ibm.com/httpserv/ihsdiag/plugin_questions.html#plugindecode
Restart your web server.