Country cannot be loaded with address->id_country

[MVDI]

Hi Everybody,

When my customers wants to register on my prestashop, they get this message : 

    1. Country cannot be loaded with address->id_country
    2. Invalid country

Prestashop version is 1.6.0.11, PHP version was 7 but I downgraded to 5.6 as I hoped it would resolve the issue, but it didn't...

Here is what I tried so far : 

- Changing `Country:name` to `country:name` in countries
- Enabling "Restrict country selections in Front Office to those covered by active carriers" (disabled it back after)
- Changing PHP version
- Changing theme
- Changing the registration process type to standard (with this option, I got a 500 error after submitting the form)

Generally speaking, the issues mentionned here didn't work for me : 
https://www.prestashop.com/forums/topic/256624-error-country-cannot-be-loaded-with-address-id_country/

Somme github issues provided files to replace, but that was for the previous versions and the changes were already included in mine. I really don't know what to do... 
Can somebody please help ?
 

[MVDI]

I may add that it happens for new registrations, did not test for stored adress.