Error when deleting item: Ensure definition type did not complete successfully. ReasonPhrase: 'Forbidden', [duplicate]

Question

I'm running SC10 XP single. I started getting the following error when deleting an item:

193784 08:17:10 ERROR Exception when executing agent aggregation/aggregator
Exception: Sitecore.XConnect.XdbCollectionUnavailableException
Message: The HTTP response was not successful: Forbidden
Source: Sitecore.Xdb.Common.Web
   at Sitecore.Xdb.Common.Web.Synchronous.SynchronousExtensions.SuspendContextLock[TResult](Func'1 taskFactory)
   at Sitecore.XConnect.Client.XConnectSynchronousExtensions.SuspendContextLock(Func`1 taskFactory)
   at Sitecore.XConnect.Client.Configuration.SitecoreXConnectClientConfiguration.Initialize(XmlNode configNode)
   at Sitecore.Configuration.DefaultFactory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert, IFactoryHelper helper)
   at Sitecore.Configuration.DefaultFactory.CreateObject(XmlNode configNode, String[] parameters, Boolean assert)
   at Sitecore.Configuration.DefaultFactory.CreateObject(String configPath, String[] parameters, Boolean assert)
   at Sitecore.XConnect.Client.Configuration.SitecoreXConnectClientConfiguration.GetClient(String clientConfigPath)
   at Sitecore.Analytics.Aggregation.XConnect.DefaultXdbContextFactory.CreateReadOnly()
   at Sitecore.Analytics.Processing.AsyncPoolScheduler`2..ctor(IAsyncProcessingPool`1 pool, IXdbContextFactory xdbContextFactory, ExpandOptions options, Int16 maxBatchSize, BaseLog log)
   at Sitecore.Analytics.Aggregation.InteractionAggregationAgent.<ExecuteCoreAsync>d__15.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Sitecore.Analytics.Core.Agent.<ExecuteAsync>d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Sitecore.Analytics.Core.AsyncBackgroundService.<ExecuteAgentAsync>d__22.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Sitecore.Analytics.Core.AsyncBackgroundService.<RunAsync>d__26.MoveNext()

Answer 1

I had exactly the same issue and after desperately trying to find a solution, the accepted answer to this question fixed it - HTTP Error 403.16 - Forbidden- Sitecore xConnect.

I hope it will help you as well. Explanation is in the answer too.