@theopenem_admin
I think I figured it out.
A registry key from TOEC-1.4.4.0 stopped the 1.5.0.0 Client from being installed.
After I deleted the key 'HKEY_CLASSES_ROOT\Installer\Products\BC4EABC65818B6043ABB1776406BC3D4' via GPO, I was able to deploy the Client again. Both deployment via GPO and deployment via OEM are working well.

Hi dblaylock,
I'm facing the same issue. I tried to update the client via GPO (I've successfully deployed the previous client via GPO as well)
It worked on some machines, but in most cases the GPO just uninstalled the older client.
I tried to install the client manually but it kept failing and refered to an older version of the installer.
On new PCs (where the old client hasn't been installed previously) it works just fine with the gpo.
have you made any progress on this topic?
cheers, Lasse