Nextcloud API groupfolders

Question

I'm trying to use nxc.get_group_folders() in pyspark to get all group_folders on Nextcloud. But every time I try to use a function from the class GroupFolder(Req, Nextcloud), an error occurs.

this is the upcoming error message:

raise JSONDecodeError("Expecting value", s, err.value) from None json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

I used the function like this:

nxc.get_group_folders()

I'm guessing, that the API_URL is wrong, but in all documentations from Nextcloud API, they use the same API_URL as I do.

My class looks like this:

class NextCloud:
    def __init__(self, endpoint, user, passwd, js=False):
        self.tojs = "?format=json" if js else ""
        self.endpoint = endpoint
        self.user_url = endpoint + "/ocs/v1.php/cloud/users"
        self.group_url = endpoint + "/ocs/v1.php/cloud/groups"
        self.share_url = endpoint + "/ocs/v2.php/apps/files_sharing/api/v1"
        self.apps_url = endpoint + "/ocs/v1.php/cloud/apps"
        self.group_folders_url = endpoint + "/ocs/v2.php/apps/groupfolders/folders"
        self.h_get = {"OCS-APIRequest": "true"}
        self.h_post = {"OCS-APIRequest": "true", "Content-Type": "application/x-www-form-urlencoded"}
        self.auth_pk = (user, passwd)


class Req(NextCloud):
# WrapRequests
    def rtn(self, resp):
        if self.tojs:  # JSON
            return resp.json()
        else:  # XML
            return resp.text

    def get(self, ur):
        res = requests.get(ur, auth=self.auth_pk, headers=self.h_get)
        return self.rtn(res)

    def post(self, ur, dt=None):
        if dt == None:
            res = requests.post(ur, auth=self.auth_pk, headers=self.h_post)
        else:
            res = requests.post(ur, auth=self.auth_pk, data=dt, headers=self.h_post)
        return self.rtn(res)

    def put(self, ur, dt=None):
        if dt == None:
            res = requests.put(ur, auth=self.auth_pk, headers=self.h_post)
        else:
            res = requests.put(ur, auth=self.auth_pk, data=dt, headers=self.h_post)
        return self.rtn(res)

    def delete(self, ur, dt=None):
        if dt == None:
            res = requests.delete(ur, auth=self.auth_pk, headers=self.h_post)
        else:
            res = requests.delete(ur, auth=self.auth_pk, data=dt, headers=self.h_post)
        return self.rtn(res)

class GroupFolders(Req, NextCloud):
    API_URL = "/apps/groupfolders/folders"
    SUCCESS_CODE = 100
    # /ocs/v2.php/apps/groupfolders/folders

    def get_group_folders(self):
        return self.get(self.group_folders_url + self.tojs)

    def create_group_folder(self, mountpoint):
        return self.post(self.group_folders_url + self.tojs, {"mountpoint": mountpoint})

etc.

Mabye someone uses this class GroupFolders and can tell me if is looks different than mine or someone had this error message as well and can help me fix it.