My Asynchronous Controller throwing intermittent error and I can't pinpoint why it's happening

I have an async controller that returns results from a collection.

Sometimes, I will get an intermittent error if I click around the page to fast.

The errors are (in the browser console):

Failed to load resource: net::ERR_HTTP2_PROTOCOL_ERROR

Or it will return this error from the front-end:

500 : Ajax request has failed 

I have tried debugging locally in Visual Studio and stepping through the code, but locally, I don't get any errors and I can't figure out why it's doing it.

I have added a line that checks to make sure the returned collection is not null, so it's not that.

Here is the controller:

[HttpGet("factapi/Factory/{factoryId}/engines")]
public async Task<IActionResult> Get(Int32 factoryId)
{
    using (var tran = Session.BeginTransaction())
    {
        var engineId = page.Assembly.Id.TrimToNull();
        if (engineId == null) return NoContent();
        var engines = await GetEngines(new Guid(engineId));
        if (engines == null)
        {
            return BadRequest();
        }
        return Ok(engines);
    }
}

This is the async method that gets the engines:

    public async Task<IEnumerable<Engine>> GetEngines(Guid id)
    {
        var engines = new List<Engine>();

        engines = await GetEngineList<Engine>(id);

        return engines.Where(t => start.LiesBetween(t.BuildTime.AddMinutes( - 7), t.BuildTime.AddMinutes((t.TimeBreak / 60) + 7)));

    }

And here is GetEngineList:

public async Task<List<T>> GetEngineList<T>(Guid id)
{
    List<T> collectionVal = default(List<T>);

    var factoryResponse = await GetList(id);
    var fullList = factoryResponse.Results;
    var settings = new JsonSerializerSettings
        {
            NullValueHandling = NullValueHandling.Ignore,
            MissingMemberHandling = MissingMemberHandling.Ignore
        };
        
    var json = JsonConvert.SerializeObject(fullList);

    var final = JsonConvert.DeserializeObject<List<T>>(json, settings);
    collectionVal = final;
    return collectionVal;
}

Another set of eyes would be welcome! :)