Hello Ian, thanks for your help, I'll dig into the content you shared. Sadly, it hasn't helped until now.
You're right when you suggest changing the "required" attribute of the fields, doing that does change the situation. I'm working with two fields, one of them an Integer and the other one (the one that causes trouble) a text field with a LookUp Table. When I set the problematic field as "not required", the resource is created correctly and the field takes the default value instead of the one I'm theoretically setting, so I believe there's a problem with the way I'm setting it. Which looks weird is that, given that it takes the default value when it's not set as required, it'd be expectable that it'd do the same thing when set as required too, allowing the creation, but instead it fails.
However, I'll keep searching and taking a look at what you shared.