I've been receiving this kind of notice alarms (nothing critical) but would be good to know why the system is generating them, could be an issue with the users?

I already tried to sync the security file on all the DMAs in the cluster but the alarms still showing.

This is the complete alarm:

Unexpected Exception [Exception from HRESULT: 0x8004028A]: The client is unknown.
(Element Subscription) (
Server stack trace:
at Interop.SLDataMiner.IDataMiner.NotifyDataMiner(String bstrUser, Int32 iType, Object varValue, Object varValue2, Object& pvarReturnValue)
at System.Runtime.Remoting.Messaging.StackBuilderSink._PrivateProcessMessage(IntPtr md, Object[] args, Object server, Object[]& outArgs)
at System.Runtime.Remoting.Messaging.StackBuilderSink.SyncProcessMessage(IMessage msg)

Exception rethrown at [0]:
at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
at Interop.SLDataMiner.IDataMiner.NotifyDataMiner(String bstrUser, Int32 iType, Object varValue, Object varValue2, Object& pvarReturnValue)
at Skyline.DataMiner.Net.DataMiner.SubscribeOnSLDataMinerElementInner(SubscribeInfo info))

Any idea on how to tackle this issue?