Can't buy at own site, personal data formulary keeps refreshing

[7150]

Hi everyone, I'm 100% new to Prestashop and a friend of mine gave me access to his site so I can learn it and develop his shop at the same time, since he doesn't have time for it anymore. 
To be honest, I believe it's fairly advanced, but there are a couple issues about the shop I'm struggling to solve.

Apparently, when trying to buy anything either as a registered user or as a guest, after entering your personal information (name, surname and e-mail) and clicking "continue", instead of going to the next step (which would be the address and such) the form resets, leaving everything blank again. Only after some attempts I was able to get to the next steps, but when trying to buy something else I have to try over and over again.

Also, the shopping cart behaves in an erratic way. I've tried adding and removing items from it and it never reflects what I did. For instance, I've tried adding to different products to it, then removing one of them and going back to mainscreen, and instead of having 1 product, I still have 2 on the cart. I did this process back and forth, and it's never the same ammount of products I should have.

All in all, my friend told me as well that he kinda got tired of it because he found Prestashop was pretty unstable and erratic, regarding the UI and the Backoffice too.

In any case, I would like to know how can I solve these issues before getting any conclussions of the like.

The URL is epamar.com.ar

The Prestashop version is 1.7.6 and is hosted on my friend's server, so I can modify things exclusively from the web site.

Cheers and thanks a lot in advance.

BTW, the site is in spanish, so here are a couple keyword translations.

Carrito: Cart

Añadir: Add

Pasar por Caja: Go to cashier/checkout

Datos personales: Personal data/info

Sr/Sra: Mr/Mrs

Nombre: Name

Apellidos: Surnames

Dirección de correo electrónico: E-mail address

Contraseña: Password

Continuar: continue