Hello,
I'm back to let you all know that the problem is solved and how I solved it.
This is a process that is in production. After it was deployed, I realized that my staff wasn't uploading or entering certain pieces of information that were later necessary for metadata, so I made them required. But I did this after some instances had already started and had been submitted without that variable filled.
In this form, in particular, there was no missing information, but this form had a collection and other members of the collection (that are not visible in this form) had missing values.
How did I find out? I looked at the console side of things in the form.
Open the form
F12 - This will open the DevTools window.
Click in the Sources Tab and on the right side, go to "Event Listener Breakpoints".
Scroll down to control and then submit (In my case it was the submit button that wasn't working, that's why I wanted the code to stop here. But you can select whichever item is causing problems).
Go back to your form and click on the Submit button (gain, this was my issue, so that's what I did)
This will show you the code related to that button. Click on "Pretty Print" and edit the code of the submit button to force a no validate (formnovalidate = "formnovalidate"). This will force that task to submit without validation.
Once I was able to submit the task, I got error messages that were super helpful to understand what was going on. This is where I was able to see that variables in other members of my collection (that were not visible in this particular form), had missed required values. Once I fixed that, the form was submitted.
In sum, I wasn't getting any feedback/errors from the user side, and only when I was able to force the submit, I was able to see the errors from the server-side.
I wish these errors were made visible without these tricks.
Valuable lesson here: don't change variables from not required to required once you have deployed your process. Or, even better, make sure you are certain of which variables should be required before deployment! :)