Why do I get a Key error when calling flask session?

I have a problem with users trying to register on my application (which is a Flask app deployed on Pythonanywhere with MongoDB Atlas).

Basically when they register they receive an email with a token that they need to click on to confirm. When they click on that link they should enter the application, but instead they receive an error.

When I check the error log, this is what I see (under "/auth/register") when it calls:

existing_user = User.get_by_email(session["email"])

return super(SecureCookieSession, self).__getitem__(key)

Exception KeyError: 'email' was thrown out.

It seems that somehow the session["email"] does not work?

This is my code:

@app.route('/confirm/<token>')
def confirm_email(token):
    try:
        email = ts.loads(token, salt="email-confirm-key", max_age=7200)

    except:
        return render_template("token_expired.html")

    user = Database.find_one(collection="users", query={"email": email})

    return redirect(url_for("register_user"))


@app.route("/auth/register", methods=["GET", "POST"])  
def register_user():

    existing_user = User.get_by_email(session["email"])

    if existing_user is None:

        email = session["email"].lower()  # our website is making a request to our app
        username = session["username"]
        companyname = session["companyname"].lower()
        password = session["password"].encode("utf-8")

        hashed_password = bcrypt.hashpw(password, bcrypt.gensalt())

        User.register(email, username, companyname, hashed_password)   
       
        session["companyname"] = companyname       

        excisting_company = Database.find_one(collection="companies", query={"email": session["email"]})

        if excisting_company is None:

            new_company = NewCompany(session["companyname"], session["email"])
            new_company.company_save_to_mongo()

        else:
            return "A company has already been registered under your email address"

    else:
        return render_template("email_already_exists.html")

    return redirect(url_for("user_results"))


@classmethod
    def get_by_email(cls, email):
        find_email = Database.find_one(collection="users", query={"email": email})
        if find_email is not None:
            return cls(email=find_email["email"], username=find_email["username"],
                       companyname=find_email["companyname"], hashed_password=find_email["password"],
                       date=find_email["date"], _id=find_email["_id"])

I cant seem to figure out why, as when I register myself it works just fine.

Your help would be greatly appreciated here, many thanks in advance.

UPDATE:

Here is where I set the session["email"]:

@app.route("/email_confirmation", methods=["GET", "POST"])
def confirmation():

    email = request.form["email"].lower() 
    companyname = request.form["companyname"].lower()
    password = request.form["password"]
    username = request.form["username"]
    session["password"] = password
    session["email"] = email
    session["companyname"] = companyname
    session["username"] = username

    existing_user = User.get_by_email(email)

    if existing_user is None:

        msg = Message("Confirm your email", sender=("hello@weliketalent.com"),
                 recipients=[email])

        token = ts.dumps(email, salt="email-confirm-key")

        confirm_url = url_for("confirm_email", token=token, _external=True)

        msg.html = render_template("email/activate.html", confirm_url=confirm_url)

        mail.send(msg)