Splunk Plugin - 401 Unauthorized

Our current setup:
VM running grafana-server with Splunk Plugin. Plugin is configured to use Basic Auth with Access Mode set to Server.

Since yesterday evening, our grafana-splunk plugin has stopped working. On the front end I am getting a 401 Unauthorized for all our splunk queries. On the server side, I can see following error message: tsdb.HandleRequest() error Could not find executor for data source type: grafana-splunk-datasource