Windows client is freezing

Hello there,

we have the following problem: When calling PAGE.RUNMODAL it can from a higher number of users (> 6), that all clients that are logged on to the same service freeze for about 2 minutes. During this time it is not possible to log on to this service. After that, everything continues as normal as if nothing had happened.
We've already checked the time synchronization. We have also changed the following value:
<add key = "ClientServicesKeepAliveInterval" value = "300" />
the NST and SQL Server running on a VMWare
In the Event Viewer is the following:
Source: mscorlib
HRESULT: -2146232828

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Type: Microsoft.Dynamics.Nav.Types.Exceptions.NavSessionTerminatedException
Message: The server session (ID: 220) stopped due to a request for stopping the session.
StackTrace: at Microsoft.Dynamics.Nav.Runtime.SessionAccessLock.ThrowSessionTerminatedException () at Microsoft.Dynamics.Nav.Service.ServiceCall.GetClientReturnValue (Action invokeCallback) at Microsoft.Dynamics.Nav.Service.NSClientCallback.FormRunModal (NavForm shape parameter NavFormRuntimeParameters ) record at Microsoft.Dynamics.Nav.Runtime.NavForm.RunModal (NavRecord, Int32 FieldNo) at Microsoft.Dynamics.Nav.Runtime.NavForm.RunModal (Boolean isInLookupTrigger, Boolean isLookup, Int32 formid, NavRecord record, Int32 FieldNo) at Microsoft .Dynamics.Nav.Runtime.NavForm.RunModal (Boolean isInLookupTrigger, Boolean isLookup, Int32 formid, NavRecord record) at Microsoft.Dynamics.Nav.BusinessApplication.Codeunit50000.ShowProductionBOMNo_Scope.OnRun () at Filter.InvokeWithFilter (NavMethodScope) at Microsoft.Dynamics .Nav.BusinessApplication.Codeunit50000.ShowProductionBOMNo (INavRecordHandle itemVar) at Microsoft.Dynamics.Nav.BusinessApplication.Codeunit50000.OnInvoke (Int32 MemberID, Object [] args) at Microsoft.Dynamics.Nav.BusinessApplication.Page99000789.a60Action1000000004a62_a45_OnAction_Scope.OnRun() at Filter.InvokeWithFilter (NavMethodScope) at Microsoft.Dynamics.Nav.BusinessApplication.Page99000789.a60Action1000000004a62_a45_OnAction ()
Source: Microsoft.Dynamics.Nav.Ncl
HRESULT: -2146233088
The customer uses NAV 2013 R2 CU 27 Build 43896
Has anyone of you an idea what's wrong.

Microsoft doesn't have an idea what can it be.

yours

Michael