The 403 error is only solved by Refreshing the page & reentering your information until it works.
When the form does submit, you’ll receive an email that says, “it will take a few days to grant you access to the website”. From here, you’ll just have to wait until you’re granted access. In the mean time, if you have Quixel Bridge, you can ‘play’ around with 50+ pre-modeled characters.
I submitted my email yesterday and this morning I was given access to online app. So, just give it time.