Hi Carlos,
What seems to have happened is that your grouping has somehow gotten nested and somewhat mangled. The form loads find as long as there is nothing in the body section (see attached), though that won't help you at all. It will require a bit of time going through the XML file itself to make sure that the grouping isn't nested, that the end tags are where they should be, etc. If you're comfortable with editing XML, this could be an option. If not, do you have an earlier version of the form that still works?
Grouping and repeats are still an experimental feature in KoBoForm and we're still looking at how to make this more reliable. I suggest creating your form first without any repeating groups so that you have an XML form to revert to if ever something like this happens again.
Best,
Tino