Can a transaction be successful in a 2-node private Stellar Network?

Question

I have a 2-node private network running, when I try to submit a transaction using an XDR generated from the Stellar Lab, it first shows up as "PENDING", then one of the node stops working.

Any idea, why would this be happening?

This is how the stellar-core.cfg looks, with modified keys:

# simple configuration for a standalone test "network"
# see stellar-core_example.cfg for a description of the configuration parameters

HTTP_PORT=11626
PUBLIC_HTTP_PORT=true
RUN_STANDALONE=true
LOG_FILE_PATH=""

NETWORK_PASSPHRASE="Standalone Test Network ; Oct 2018"

NODE_SEED="SCNNL6IKBNRINHHXE3W2SO6RLKIU2Q2WHCGBQEZIW74SPBTZJTNBVO7M zagg-node01"
NODE_IS_VALIDATOR=true

KNOWN_PEERS=["127.0.0.1:11635"]

#DATABASE="postgresql://dbname=stellar user=postgres password=password host=localhost"
#DATABASE="sqlite3://stellar.db"
DATABASE="postgresql://dbname=stellar_node01_db host=localhost user=stellar_node_user password=testdb"


#Bucket directory path
BUCKET_DIR_PATH = "buckets"

PEER_PORT=11625

COMMANDS=["ll?level=trace"]

FAILURE_SAFETY=0
UNSAFE_QUORUM=true
#The public keys of the Stellar testnet servers
[QUORUM_SET]

THRESHOLD_PERCENT=100
VALIDATORS=["$zagg-node01"]

[HISTORY.vs]
get="cp tmp/stellar-core/history/vs/{0} {1}"
put="cp {0} tmp/stellar-core/history/vs/{1}"
mkdir="mkdir -p tmp/stellar-core/history/vs/{0}"