Instagram doesn't allow proton emails for registration.
Fuck you Mark. Fuck you buddy.
But yeah, the first error message I got was that the code sent to my e-mail was expired. I thought huh, I guess that is a little fair because usually it takes a while for me to get the notification on my phone. Even then, it's not so long to the point where it'd get expired. At least 3-5 seconds max. There wasn't even a countdown or anything either.
Then I tried again, they sent another code, this one being a little faster, and then I got the error message saying that there was a problem signing up. Try again. Tried again, same error message. I tried with my google account, and it worked.
So yeah they've clearly blatantly done something to block Proton Mail, I guess they hate not having your personal information or something. Still though, had to sign up since being a creative these days require you to have Instagram...