If you try to use on site collections NOT hanging off of the root, the links are jacked up.
Find the features.xml files for the two following folders (found C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\FEATURES)
<UrlAction Url="/_layouts/FBA/Management/UsersDisp.aspx" />
<UrlAction Url="~sitecollection/_layouts/FBA/Management/UsersDisp.aspx" />
Basically, they left out the ~sitecollection
While you're at it, find the features XML for
FBAManagementSelf and ensure the Last node is remarked or removed