I have had exactly the same problem - I can push the 12.1.5 client to anything but the SEPM host server.
SymHelp is all green - more so than it's ever needed to be before on any of several SEPM managers I've set up with versions prior to 12.1.5. As did kf-ict, I've recheked and rechecked everything and it's all been good.
While the "Save Package" option did work for me, too, it's a work around, not a fix. I say this becaue remote push is suppposed work.