Error in Event Viewer about Logs

I have this message in Event Viewer:

"Error sending log: System.NullReferenceException: Object reference not set to an instance of an object.
   at OutSystems.HubEdition.RuntimePlatform.DBRuntimePlatform.CreateVisitLog(IDbConnection conn, CustomLog log)
   at OutSystems.HubEdition.RuntimePlatform.Log.LoggerSynchronous.Log(CustomLog log)
   at OutSystems.HubEdition.RuntimePlatform.Log.RuntimeLogger.LogVisit(DateTime instant, String visitorUID, String visitUID)"

This is generated by a Platform 7.0 and it has been appering in Event Viewer for the last day or so.
I've restarted the services, but the error persists.
I have enought space on hard drive in both app server and DB server.

Can you give me some clues as to what might be causing this?
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