Update: I cleared the cache and restarted OH twice. This made the problem go away!
I installed the Trådfri GW, a remote control and an RGB light a couple of days ago, using the Paper UI. Everything went well and was quite easy to do. The light works as expected. But the GW generates an error message once every minute. See below. The GW version is 1.10.29 and I am running OH 2.5.2 release version.
2020-03-23 10:05:54.839 [WARN ] [.eclipse.californium.core.CoapClient] - exception while handling response
java.lang.IllegalStateException: Not a JSON Array: {"9078":1,"9023":"0.tradfri.pool.ntp.org","9059":1584954354,"9060":"2020-03-23T09:05:54.042784Z","9062":0,"9066":5,"9061":0,"9029":"1.10.29","9106":0,"9075":0,"9081":"7e3765520440008c","9082":false,"9093":0,"9083":"N/A","9054":0,"9055":0,"9069":1584719623,"9071":1,"9072":3,"9073":29,"9074":1,"9092":0,"9200":"5c936dbf-037d-4e92-b92a-f43ee04945f9","9076":10,"9079":0,"9077":25,"9103":"","9080":60,"9118":0,"9105":0,"9107":0,"9201":2,"9202":1584719800}
at com.google.gson.JsonElement.getAsJsonArray(JsonElement.java:107) ~[?:?]
at org.openhab.binding.tradfri.internal.handler.TradfriGatewayHandler.onUpdate(TradfriGatewayHandler.java:314) ~[?:?]
at org.openhab.binding.tradfri.internal.TradfriCoapHandler.onLoad(TradfriCoapHandler.java:73) ~[?:?]
at org.eclipse.californium.core.CoapClient$MessageObserverImpl.deliver(CoapClient.java:1466) ~[bundleFile:?]
at org.eclipse.californium.core.CoapClient$MessageObserverImpl$1.run(CoapClient.java:1448) [bundleFile:?]
at org.eclipse.californium.core.CoapClient.execute(CoapClient.java:1289) [bundleFile:?]
at org.eclipse.californium.core.CoapClient$MessageObserverImpl.succeeded(CoapClient.java:1444) [bundleFile:?]
at org.eclipse.californium.core.CoapClient$MessageObserverImpl.onResponse(CoapClient.java:1434) [bundleFile:?]
at org.eclipse.californium.core.coap.Request.setResponse(Request.java:711) [bundleFile:?]
at org.eclipse.californium.core.network.EndpointManager$ClientMessageDeliverer.deliverResponse(EndpointManager.java:272) [bundleFile:?]
at org.eclipse.californium.core.network.stack.BaseCoapStack$StackTopAdapter.receiveResponse(BaseCoapStack.java:210) [bundleFile:?]
at org.eclipse.californium.core.network.stack.AbstractLayer.receiveResponse(AbstractLayer.java:89) [bundleFile:?]
at org.eclipse.californium.core.network.stack.ExchangeCleanupLayer.receiveResponse(ExchangeCleanupLayer.java:93) [bundleFile:?]
at org.eclipse.californium.core.network.stack.ObserveLayer.receiveResponse(ObserveLayer.java:150) [bundleFile:?]
at org.eclipse.californium.core.network.stack.BlockwiseLayer.receiveResponse(BlockwiseLayer.java:676) [bundleFile:?]
at org.eclipse.californium.core.network.stack.ReliabilityLayer.receiveResponse(ReliabilityLayer.java:305) [bundleFile:?]
at org.eclipse.californium.core.network.stack.AbstractLayer.receiveResponse(AbstractLayer.java:89) [bundleFile:?]
at org.eclipse.californium.core.network.stack.BaseCoapStack.receiveResponse(BaseCoapStack.java:139) [bundleFile:?]
at org.eclipse.californium.core.network.CoapEndpoint$1.receiveResponse(CoapEndpoint.java:277) [bundleFile:?]
at org.eclipse.californium.core.network.UdpMatcher$4.run(UdpMatcher.java:399) [bundleFile:?]
at org.eclipse.californium.elements.util.SerialExecutor$1.run(SerialExecutor.java:276) [bundleFile:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_232]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_232]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180) [?:1.8.0_232]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293) [?:1.8.0_232]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_232]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_232]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_232]