Node errors

"double free or corruption (fasttop)" error in log

You need to update Ubuntu and restart the server.

"method not found" error in log

This is an empty request and doesn't interfere with the server.

Private key error

Make sure that a private key was formated to HEX. A correct private key in HEX-format starts from "307" beside "0x" in the beginning.
The page for validation private key -

Node works properly but doesn't get the reward

Make sure that this server have enough RPS/QPS. Minimum for it - 2000.