Hey Jacob, it looks like we're having some issues with our email provider and it's not sending out invitation emails (and giving an error that shows up as an error 500 for you).
I'm reaching out to them now to figure out what's going on and hope to get it sorted out soon, but in the meantime you should be able to skip the invite stage altogether by sending an invite to someone who already has an account on Notebook.ai with that exact email. When an invite is created for an account that already exists, it's supposed to automatically accept and instantly give them collaboration access. Depending on the error, there's a small chance this won't work, but it might be a workaround if the email provider takes some time to get back to me.
If you'd like, feel free to PM me the universe and the email you're trying to invite and I can link an existing account to it as a collaborator also.
Sorry about the inconvenience; I hope to have it sorted out soon!