SendInBlue (Brevo) plugin throws 400 error on some customers when tracking events

I am using the SendInBlue plugin (now renamed to Brevo) to track the events of my customers.
However, I notice that at some points the events of the customers, like cart_updated or order_completed, are not registered in SendInBlue.
The plugin then shows this error in the nopCommerce log:
System.Net.Http.HttpRequestException: Response status code does not indicate success: 400 (Bad Request).
   at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode()
   at Nop.Plugin.Misc.Sendinblue.Services.MarketingAutomationHttpClient.RequestAsync[TRequest](TRequest request)
   at Nop.Plugin.Misc.Sendinblue.Services.MarketingAutomationManager.HandleShoppingCartChangedEventAsync(ShoppingCartItem cartItem)

It is not with all customers, but only sometimes. Any idea what is causing this and if it can be resolved?

It looks like this is related to customers who are using the site as a guest, instead of a registered user as I noticed that guest users are not tracked in the marketing automation of Brevo, even if they place an order.

What nopCommerce version and what plugin version do you use?

NopCommerce version 4.60.2 and plug-in version 3.56

Hi. have you solved it? We have this in 4.50