You might want to choose a default outflow for the user task in case the instance matches none of the requirements, you might see the instance is terminated but without any description; and from my view of point, you need to add an outflow for the above service task. You might click the validate button (which is a tick button located at the right top next to save button) to see whether there is something we have accidentally missed when designing the process.

Above is just some thoughts which may not solve your problem because we don't know the root cause yet. Your problem is when the timer fires, the user task terminated, right? May I ask which version of Forms are you using? And could you provide more information how the user task is terminated, like the user task didn't select a form, or the user task wasn't assigned, etc.? For example, you may go to the instances metric page of that process, click on the dropdown menu and see the error log. Because based on your design, the timer shouldn't terminate the user task. Correct behavior for it should be the timer will run every 10 minutes to fire a service task as long as user task is active. Thus we need the reason why the user task is terminated to further analyze the issue you have.