Http failure response for https://myvra.vclass.local/abx/api/resources/actions/8a748003700e0ab601700e2577100000?projectId=eacf... 500 OK
vRA8 is not good about documenting the errors it receives and the code 500 is a generic internal system error. You could connect to the pod and pull the logs (easier to just create the vRA log bundle (vracli log-bundle), but from what I have seen the errors are not descriptive either.
Could it be that your action is part of a subscription, if so that would have to be removed first. It is also possible that your action is part of another flow action, in which case it would have to be removed from the action flow as well.