Welcome to the Atlassian Developer Community, @mariowitdoek !
I checked the Atlassian Python API wrapper’s implementation and in jira.py they are using a deprecated field name when assigning issues (see Assign issue docs for reference).
Here you can pass the assignee as a field. For reference, this is a sample body parameter when setting the assignee field while using Edit issue REST API. For this library, I assume you only need the assignee part.
I still had this problem today, even with the current version of the API updated. Today it checks for “self.cloud” bool, but this this is false by default. To fix this, just initialize your Jira object with “cloud = True”
Checking their documentation, you are correct, you have to explicitly set cloud to True when initializing the Jira object.
# Obtain an API token from: https://id.atlassian.com/manage-profile/security/api-tokens
# You cannot log-in with your regular password to these services.
jira = Jira(
url='https://your-domain.atlassian.net',
username=jira_username,
password=jira_api_token,
cloud=True)