I found this in dmddevice.log
[1] 2021/04/07 19:10:02.168 ERROR | System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
at System.Collections.Generic.Dictionary`2.ValueCollection.Enumerator.MoveNext()
at LibDmd.Output.Virtual.AlphaNumeric.VirtualAlphanumericDestination.Dispose() in C:\Development\dmd-extensions\LibDmd\Output\Virtual\AlphaNumeric\VirtualAlphanumericDestination.cs:line 311
at LibDmd.RenderGraph.Dispose() in C:\Development\dmd-extensions\LibDmd\RenderGraph.cs:line 1052
at LibDmd.RenderGraphCollection.<>c.<Dispose>b__17_0(RenderGraph graph) in C:\Development\dmd-extensions\LibDmd\RenderGraphCollection.cs:line 138
at System.Collections.Generic.List`1.ForEach(Action`1 action)
at LibDmd.RenderGraphCollection.Dispose() in C:\Development\dmd-extensions\LibDmd\RenderGraphCollection.cs:line 138
at LibDmd.DmdDevice.DmdDevice.Close() in C:\Development\dmd-extensions\LibDmd\DmdDevice\DmdDevice.cs:line 568
at PinMameDevice.DmdDevice.Close()
at FlexDMD.DMDDevice.Close()
at FlexDMD.FlexDMD.ShowDMD(Boolean show)
at FlexDMD.FlexDMD.set_Color(Color value)
--- End of inner exception stack trace ---
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at System.Reflection.RuntimePropertyInfo.SetValue(Object obj, Object value, BindingFlags invokeAttr, Binder binder, Object[] index, CultureInfo culture)