UMAPI call returns 504 when searching by domain

I'm using the Python UMAPI client (installed from PyPi).  

If I do a generic query for all of my users, I'm ok..

query = umapi_client.UsersQuery(conn)

But if I try to limit the query to one of our domains...

query = umapi_client.UsersQuery(conn, in_domain='example.edu')

...the API returns a 504 error...

DEBUG: Starting new HTTPS connection (1): ims-na1.adobelogin.com:443
DEBUG: https://ims-na1.adobelogin.com:443 "POST /ims/exchange/jwt/ HTTP/1.1" 200 766
DEBUG: Starting new HTTPS connection (1): usermanagement.adobe.io:443
DEBUG: https://usermanagement.adobe.io:443 "GET /v2/usermanagement/users/IMS0123456789@AdobeOrg/0?domain=example.edu&directOnly=True HTTP/1.1" 504 0
WARNING: UMAPI timeout...service unavailable (code 504 on try 1)

I get the same response if I try invoking the endpoint using Postman:

https://usermanagement.adobe.io:443/v2/usermanagement/users/IMS0123456789@AdobeOrg/0?domain=example.edu

Any suggestions?

Liam