vCenter Server 8.0 U3
This is a known issue impacting vCenter Server 8.0 U3 and it is fixed in vCenter version 8.0U3e
Workaround:
To workaround the issue, clear the Browser cache & cookies and attempt the login again
vCenter Server 8.0 U3 へのパッチ適用またはアップグレード後に vSphere Client や VAMI のログインボタンがグレーアウトする