If this is a corny keg the spring loaded thing with claws opposite the Oring is called (at least in some places) a poppet valve.
The claw end should be sitting on the flared end of the out tube if everything was lined up good on re-assembly.
For today you can maybe take the post off, push the poppet out and lube it up good with some keg lube. Or just leavgfe it tapped. With a black connector on their the poppet will be down/open and the beer tubing will be at the same pressure as the keg.
I think the leaking out post poppet is a separate problem from the foam at the tap. How was the beer carbonated, how many volumes, and how long /what diameter is the hose from your keg to your tap?