System.NullReferenceException sometimes

It seems the manager starts frequently crashing with exception below. I think it somehow can be connected when I added new restricted user. This is server edition under Ubuntu Linux .

Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object
  at System.Net.HttpConnection.get_RemoteEndPoint () [0x00000] in <bd46d4d4f7964dfa9beea098499ab597>:0
  at System.Net.HttpListenerRequest.get_RemoteEndPoint () [0x0000b] in <bd46d4d4f7964dfa9beea098499ab597>:0
  at HttpFramework.HttpRequest..ctor (System.Net.HttpListenerRequest request, System.String prefix) [0x000ad] in <19bedf15aa0f4284a9957cbc7b97b922>:0
  at HttpFramework.HttpServer.Worker () [0x0007b] in <19bedf15aa0f4284a9957cbc7b97b922>:0
  at System.Threading.ThreadHelper.ThreadStart_Context (System.Object state) [0x00017] in <8f2c484307284b51944a1a13a14c0266>:0
  at System.Threading.ExecutionContext.RunInternal (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx) [0x0008d] in <8f2c484307284b51944a1a13a14c0266>:0
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx) [0x00000] in <8f2c484307284b51944a1a13a14c0266>:0
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state) [0x00031] in <8f2c484307284b51944a1a13a14c0266>:0
  at System.Threading.ThreadHelper.ThreadStart () [0x0000b] in <8f2c484307284b51944a1a13a14c0266>:0
[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object
  at System.Net.HttpConnection.get_RemoteEndPoint () [0x00000] in <bd46d4d4f7964dfa9beea098499ab597>:0
  at System.Net.HttpListenerRequest.get_RemoteEndPoint () [0x0000b] in <bd46d4d4f7964dfa9beea098499ab597>:0
  at HttpFramework.HttpRequest..ctor (System.Net.HttpListenerRequest request, System.String prefix) [0x000ad] in <19bedf15aa0f4284a9957cbc7b97b922>:0
  at HttpFramework.HttpServer.Worker () [0x0007b] in <19bedf15aa0f4284a9957cbc7b97b922>:0
  at System.Threading.ThreadHelper.ThreadStart_Context (System.Object state) [0x00017] in <8f2c484307284b51944a1a13a14c0266>:0
  at System.Threading.ExecutionContext.RunInternal (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx) [0x0008d] in <8f2c484307284b51944a1a13a14c0266>:0
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state, System.Boolean preserveSyncCtx) [0x00000] in <8f2c484307284b51944a1a13a14c0266>:0
  at System.Threading.ExecutionContext.Run (System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, System.Object state) [0x00031] in <8f2c484307284b51944a1a13a14c0266>:0
  at System.Threading.ThreadHelper.ThreadStart () [0x0000b] in <8f2c484307284b51944a1a13a14c0266>:0

Not sure why this would start happening but the latest version (18.1.64) should be fixing this issue.

Thanks for the quick reply!