2020-07-24 18:28:22,722 ERROR o.g.b.p.OutputBufferProcessor [outputbuffer-processor-executor-1] Error in output [class org.graylog2.plugins.slack.output.SlackMessageOutput].java.lang.RuntimeException: Could not send message to Slack.        at org.graylog2.plugins.slack.output.SlackMessageOutput.write(SlackMessageOutput.java:94) ~[?:?]        at org.graylog2.buffers.processors.OutputBufferProcessor$1.run(OutputBufferProcessor.java:191) [graylog.jar:?]        at com.codahale.metrics.InstrumentedExecutorService$InstrumentedRunnable.run(InstrumentedExecutorService.java:181) [graylog.jar:?]        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_262]        at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_262]        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_262]        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_262]        at java.lang.Thread.run(Thread.java:748) [?:1.8.0_262]Caused by: org.graylog2.plugins.slack.SlackClient$SlackClientException: Unexpected HTTP response status 403        at org.graylog2.plugins.slack.SlackClient.send(SlackClient.java:74) ~[?:?]        at org.graylog2.plugins.slack.output.SlackMessageOutput.write(SlackMessageOutput.java:92) ~[?:?]        ... 7 more