Call me greedy, but Speckle crashes if I have 2 add-ins open at once. In this case I have the Revit and Dynamo plug-ins open (looking at separate models). If I have the Dynamo plug-in open I get a “cannot execute external command” error within Revit. If the other way round, Dynamo won’t place speckle nodes and crashes.
I could replicate with Revit 2019 & Dynamo 2.0.3, but doesn’t seem to occur with Revit 2020 or above!
Not sure if possible, but you could try to run a newer version of Dynamo or just switch to 2020…