If you see MSI log entry Custom Action Server rejected - Wrong Context. logged during installs on 64 bit platforms, you may have an incorrectly registered custom action server. This log entry will be preceeded by the log entry Hello, I'm your 64bit Elevated custom action server.. This can be caused by the 32 bit custom action server incorrectly being registered as the 64 bit custom action server.
Digging deeper, check the values of the following registry values on the x64 machine...
HKLM\Software\Microsoft\Windows\CurrentVersion\Installer ! MsiExecCA32
HKLM\Software\Microsoft\Windows\CurrentVersion\Installer ! MsiExecCA64
...to see if they are pointing to the same path location. If they are, you've got a misregistered custom action server as, normally, the MsiExecCA32 should reference the syswow64 location.
You can easily fix this up by running
Generally, this should be a rare occurance but given that the market is just beginning to convert to 64 bit machines, it seemed like something that may occur more often as 64 bit adoption increases.