Basic Authentication within a POST-request using HTTPClient

I want to call a service with HTTPpost behind a BasicAuth using
HTTPClient like this:

client =
body = { ... }
client.set_auth(domain, user, password) (domain, body)

Receive a 401!
If I try it with GET, it's perfectly working, so I know the user/
password is correct.

It seems, using POST, the set_auth-method is not called.

Does anyone has an idea or solved this problem?

thx g