Thanks for everyone's help!
I guess we can close this.

I just deployed Grafana on Docker and everything seems to be working, I installed the same plugins on the same version of Grafana you are using (5.2.0). Could you try deploying the container without swarm? If it works, then the problem could be somewhere in your configuration, this is the command I used docker run -e "GF_SERVER_PROTOCOL=http" -e "GF_INSTALL_PLUGINS=grafana-clock-panel,grafana-simple-json-datasource,grafana-piechart-panel,percona-percona-app,raintank-kubernetes-app,ntop-ntopng-datasource,natel-plotly-panel,camptocamp-prometheus-alertmanager-datasource,vonage-status-panel,fetzerch-sunandmoon-datasource,radensolutions-netxms-datasource" -d --name=grafana -p 3000:3000 grafana/grafana:5.2.0
Also I noticed that Grafana version 5.2.1 is available, maybe that will solve the issue.

Regards,
Eriks

My normal install is automated using Docker Compose and bind mounting:

# Create the bind mount for Grafana
mkdir -p /opt/grafana
cp ldap.toml /opt/grafana/
chown -R 472:472 /opt/grafana
chmod a+rw -R /opt/grafana

# docker-compose.yml
version: '3.4'

services:

  # Grafana: https://hub.docker.com/r/grafana/grafana/
  grafana:
    image: grafana/grafana:5.2.0
    hostname: grafana
    environment:
      - GF_SERVER_PROTOCOL=http
      - GF_AUTH_LDAP_ENABLED=true
      - GF_AUTH_LDAP_CONFIG_FILE=/var/lib/grafana/ldap.toml
      - GF_INSTALL_PLUGINS=grafana-clock-panel,grafana-simple-json-datasource,grafana-piechart-panel,percona-percona-app,raintank-kubernetes-app,ntop-ntopng-datasource,natel-plotly-panel,camptocamp-prometheus-alertmanager-datasource,vonage-status-panel,fetzerch-sunandmoon-datasource,radensolutions-netxms-datasource
    volumes:
      - type: bind
        source: /opt/grafana
        target: /var/lib/grafana
    networks:
      - logging-net1
    ports:
      - mode: host
        target: 3000
        published: 3000
    deploy:
      labels:
        local.description: "Grafana"

networks:
  logging-net1:

# Deploy to Docker
############
# Run Graylog on the Swarm
docker stack deploy -c docker-compose.yml grafana
# Verify
docker service ls | grep grafana

Then login to Grafana and try to add a new NetXMS data source and boom (see attached screen capture).
Note: The same docker-compose.yml above works fine with older versions of the Grafana Docker image, only changing one line:
    image: grafana/grafana:5.1.2  <--works
    image: grafana/grafana:5.1.3  <--works
    image: grafana/grafana:5.2.0  <--fails as described

Could you specify how you installed the plugin? Was it via the Grafana client using the command grafana-cli plugins install radensolutions-netxms-datasource or did you clone the GitHub repository of the plugin?

I've just tested the plugin with the latest version of Grafana (5.2.0) and all seems to be working for me. Could you please reinstall the NetXMS plugin and see if it changes anything?

Regards,
Eriks