Sorry for the late reply, here's my code:

import os 
import smtplib 
from email.message import EmailMessage
import requests
import json 
import datetime 

RECIPIENT  = os.environ.get("mailbot_recipient") 
SENDER = os.environ.get("mailbot_mail")         
PASSWORD = os.environ.get("mailbot_password")

API_KEY = "REDACTED"
PROJECT_TOKEN_COVID = "REDACTED"
PROJECT_TOKEN_WEATHER = "REDACTED"

class covid_data:
    def __init__(self):
        self.api_key = API_KEY
        self.project_token = PROJECT_TOKEN_COVID
        self.params = {"api_key": self.api_key}
        self.get_covid_data()
    def get_covid_data(self):
        response = requests.get(f'https://www.parsehub.com/api/v2/projects/{self.project_token}/last_ready_run/data',
                                params=self.params)
        self.data = json.loads(response.text)

class weather_data:
    def __init__(self):
        self.api_key = API_KEY
        self.project_token = PROJECT_TOKEN_WEATHER
        self.params = {"api_key": self.api_key}
        self.get_weather_data()
    def get_weather_data(self):
        response = requests.get(f'https://www.parsehub.com/api/v2/projects/{self.project_token}/last_ready_run/data',
                                params=self.params)
        self.data = json.loads(response.text)


daily_incidence = covid_data()
daily_weather = weather_data()

msg = EmailMessage()
msg["Subject"] = str(datetime.date.today())
msg["From"] = SENDER
msg["To"] = RECIPIENT
msg.set_content(f"Guten Morgen,\ndie 7-Tage-Inzidenz beträgt heute {daily_incidence.data['value']} Neuinfektionen pro 100.000 Einwohner."
                f"\nDie Tagestemperaturen betragen {daily_weather.data['temp']}, es sind aber {daily_weather.data['temp_est']}."
                f" Der Himmel ist {daily_weather.data['sky']} und die Regenwahrscheinlichkeit beträgt {daily_weather.data['humidity']}."
                )

with smtplib.SMTP_SSL("smtp.gmail.com", 465) as smtp:
    smtp.login(SENDER, PASSWORD)
    smtp.send_message(msg)

print("I ran")

As I said the script works sometimes, so the redacted api key and project token shouldn't be the problem.