Why is this topic resolved? It’s not resolved…
I have also this problem and it’s indeed related to WooCommerce.
But it looks like they point to eachother where the problem lies.
In my case i had no problems since a couple days and there was only an update from Ninja forms, and not WooCommerce.
It doesn’t matter if you install SMTP or set the form as redirect/succes message after sending.
But this is very bad for business!