Some time we face this problem because of setup env.
unregistration of Service and solution form SSO is not successful so it can be removed properly.
Workaround:
1) Workaround is to re-start the vsphere web clien installation after the vsphere web clien rolls back. The older solution user that remains after the workaround can be removed manually.
2.) Manually unregistered solution (vsphere web client) from SSO then re-install vsphere web client 5.5. See the steps to unregistered sloution from SSO. VMware KB: Cannot remove obsolete solution users from the Lookup Service using the vSphere Web Client