cant run function, because signal only works in main thread of the main interpreter?

Question

I'm trying to build an API using Flask. If the endpoint /register/string:token is retrieved and the token is in a defined list, the function register_user(username) should be executed. This function sends a command to another server using the rcon protocol. However, as soon as the function is called, I get the following error: ValueError: signal only works in main thread of the main interpreter. How can I fix the problem? Or, how can I run the function in the main thread?

My code looks something like this (Note: The script runs fine on Windows):

from flask import Flask, send_file
from mcrcon import MCRcon as rcon

import configparser

api = Flask(__name__)


# @api.route('/register/<string:token>', methods=['GET'])
def register(token):
    cache = utils.load_cache()
    if token in list(cache['register_tokens'].keys()):
        utils.register_user(username=cache['register_tokens'][token]) # problem!
        del cache['register_tokens'][token]
        utils.update_cache(obj=cache)
        return {'status':'registration successful'}
    else:
        return {'status':'registration failed'}



class utils:
    @staticmethod
    def load_cache(file='../cache.pkl'):
        ...

    @staticmethod
    def update_cache(obj, file='../cache.pkl'):
        ...

    @staticmethod
    def clear_cache():
        ...

    @staticmethod
    def register_user(username):
        with rcon(config['global']['server_ip'], config['global']['server_rcon_secret']) as console:
            r = console.command(...)        
            console.disconnect()



if __name__ == '__main__':
    config = configparser.ConfigParser()
    config.read('../config.conf')

    if bool(config['discordbot.py']['enable'])==True:
        api.add_url_rule('/register/<string:token>', 'register', register)

    api.run(host=config['global']['server_ip'], debug=False, use_reloader=False)