No joy with 5.15, different error text though.

On another attempt with an older system
RobotStudio .NET exception: The given key was not present in the dictionary.
============================================
Base exception: KeyNotFoundException
The given key was not present in the dictionary.
Server stack trace:
at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
at RobotStudio.Services.Controller.VcConfigurationData.GetSingleType(String singleName)
at RobotStudio.Services.Controller.VcConfigurationData.GetMechUnitItem(String mechUnit, Int32 itemNo)
at RobotStudio.Services.Controller.VcExternalAxisConfigurationData.GetMechUnitInfos()
at RobotStudio.Services.Controller.VcConfigurationData.<>c__DisplayClass1.b__0()
at System.Threading.Tasks.Task`1.InvokeFuture(Object futureAsObj)
at System.Threading.Tasks.Task.InnerInvoke()
at System.Threading.Tasks.Task.Execute()
Exception rethrown at [0]:
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccess(Task task)
at RobotStudio.Services.Controller.ControllerService.d__38.MoveNext()
Exception rethrown at [1]:
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccess(Task task)
at RobotStudio.Services.Controller.ControllerService.d__2b.MoveNext()
Exception rethrown at [2]:
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccess(Task task)
at ABBInternal.Controller.StationVirtualControllerManager.d__10.MoveNext()
RobotStudio .NET exception: No response from controller.
============================================
Base exception: RobApiException
No response from controller.
Server stack trace:
Exception rethrown at [0]:
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccess(Task task)
at RobotStudio.Services.Controller.ControllerService.d__2b.MoveNext()
Exception rethrown at [1]:
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccess(Task task)
at ABBInternal.Controller.StationVirtualControllerManager.d__10.MoveNext()