Is it possible to restart a step in a running workflow?
No, currently you cannot do it on a Nintex Workflow. If you need this kind of feature on your workflow it 'd be considered as part of your workflow design.
For example add a flexi task (or another action or condition) when you need to goto again to your step (perhaps a state on a state machine)
Depends on your goal. In the current running workflow you can use a state machine and run multiple times through one state.
But in case your current workflow includes a few steps and you run into an error and you'd like to start from 'action 5' instead of 'action 1'... this is not possible in one workflow instance.
Probably the easiest way to accomplish this is using a state machine and storing the state you are in, in a variable. This way you can write the status of a workflow to your item (or to a seperate list, depends on what you prefer as being a more clean solution). When doing this, you can let your workflow check in what state it was en you can "restart" from a certain state (basicly by letting the workflow take the correct route).
Here is an excellent Blog Post The Art of State Machines
You can use this template to help you to restart in specific step.
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