Got an error when saving a document in a MERN app - reactjs

It's happening this: sometimes, when I try to insert a new document (such as a new user,product or sale register) in my MERN app, it works fine, but some other times I'm getting the following in my console:
"Failed to load resource: the server responded with a status of 400 (Bad Request)"
"Uncaught (in promise) Error: Request failed with status code 400"
Also, in previous versions of this app, those things which worked fine previously doesn't work now.
When I restart my computer, sometimes I'm abled to add, for example, a new user, but after that, I'm not. That is, sometimes works and sometimes doesn't.
I think it must be a problem with MongoDB, maybe my computer is out of resources.
I'm using mongoose and axios.
Here's my project in gitLab:
https://gitlab.com/leandrocaplan/final-programacion-iii
A little bit of spanish lesson to understand a little better:
"Usuario" means user, so on, at the section "Alta de empleado (Employee create)", we must create a new user. "Venta" means sale, "Compra" means puchrase, and "Producto" means product.
Does anyone have an idea of what can be happening?
Thank you very much
Leandro

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Console error
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