Unexpected token 'A', "A server e"... is not valid JSON

Hello dear Brave support team,

Even though this is not stricktly Brave-related, it is a serious technical problem that I keep facing.

The story is, I am trying to subscribe to a newsletter of a musical artist I really like. It required me to give a lot of private data in order to access the newsletter and its features. Most sensitive out of all those data was the compulsory requirement of my telephone number. As you might’ve guessed, I wasn’t too keen on giving my telephone number away, so I went on some random site, found a fake telephone number and put it in the blank spot, all happy thinking how I beat the system. But then a message appeared saying: “Once you receive an SMS message with a code in it, please use it to fill out this new blank spot”.
Oh snap. I definitely did not beat the system.
Defeated, I see that there is no other thing left for me to do than to give my telephone number. I do that, and fill out all the other required information, however, now I keep facing a new problem. Every single time that I try to register, I get this one error, even though all of my data has been correctly put in:

Unexpected token ‘A’, “A server e”… is not valid JSON

What the hell is this? I searched the internet for a solution, but… No one really has any answers. I’ve only seen some very long videos of people programing, and that’s not my field of expertise at all.

Can you help me, oh fellow Brave support team? I really want to follow this musical artist, he’s very important to me