I have figured this out. When we installed 2007SP1 add-ins, we uninstalled 6.0, installed 2007SP1, and ran resetevclient.exe. Running resetEVClient does not register valkyrie.dll properly, and the close button breaks as a result. Registering this dll manually fixes the problem.
Interestingly, if you have 2007SP1 installed and working, running resetevclient.exe will break the close button. You will have to manually register it again.