NAV 2009 RTC Crashes randomly with error

Dear community,

one of my clients is experiencing random crashes of the NAV Service tier (WebService). The frequency is 3-4 times a day. They are using NAV2009 R2 with classic client and extensive usage of Web Services via middle tier.
The crash results in the following entries in the event log:

Message: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
StackTrace:
     at Microsoft.Dynamics.Nav.Runtime.NativeMethods.AdapterCloseConnection(IntPtr connectionHandle, Boolean withGrace)
     at Microsoft.Dynamics.Nav.Runtime.NavConnection.CloseConnection(Boolean withGrace)
     at Microsoft.Dynamics.Nav.Runtime.NavConnection.Dispose(Boolean disposing)
     at Microsoft.Dynamics.Nav.Runtime.ConnectionPool`1.Scavenger(Object unused)
Source: Microsoft.Dynamics.Nav.Ncl
 
Service: MicrosoftDynamicsNavWS$XXX
User: XXX\ZZZ
Type: System.AccessViolationException
Message: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
StackTrace:
     at Microsoft.Dynamics.Nav.Runtime.NativeMethods.AdapterCloseConnection(IntPtr connectionHandle, Boolean withGrace)
     at Microsoft.Dynamics.Nav.Runtime.NavConnection.CloseConnection(Boolean withGrace)
     at Microsoft.Dynamics.Nav.Runtime.NavConnection.Dispose(Boolean disposing)
     at Microsoft.Dynamics.Nav.Runtime.ConnectionPool`1.Scavenger(Object unused)
     at System.Threading._ThreadPoolWaitCallback.WaitCallback_Context(Object state)
     at System.Threading.ExecutionContext.runTryCode(Object userData)
     at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
     at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
     at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
     at System.Threading._ThreadPoolWaitCallback.PerformWaitCallbackInternal(_ThreadPoolWaitCallback tpWaitCallBack)
     at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback(Object state)
Source: Microsoft.Dynamics.Nav.Ncl

One of the suspicions is that the client is running close to the license limit on the number of the full users (they have however enough light users). However I have always believed that these are not checked for web services?

Please let me know any suggestions of what may be wrong.

Thanks,
Igor