Installed Jenkins, using a pre-existing "hudson" HUDSON_HOME from Hudson 1.377
My users work, and it obviously recognises them because I cannot add another user/group with the same name as those I know I have in there. But the user/group column entry shows up as "ERROR" (underlined, a link) for all of them; Clicking the ERROR link pushes this text in to the table cell:
HTTP Status 404 -
type Status report
message
descriptionThe requested resource () is not available.
GlassFish Server Open Source Edition 3.1
an existing "Hudson"

See attached screenshot.