Same issue here regarding the “Private key incorrect for xxxxx” message. I’ve not touched the private key so am pretty sure it’s not changed in any way.
I had everything working fine, I was running 0.6.1 with WordPress 4.1.x and could update plugins OK. I then updated WP core to 4.2.2, the update applied fine but since doing so I can’t seem to add/remove/update anything, I just get the private key error.