Cannot authenticate with usermnames containing 'ä','ö','ü'

[queso1307]

I am trying to connect to a Windows2012r2 Server running IIS8 from my Ubuntu 16.04 machine.
My username contains a 'ä'.
When calling:

session.auth = HttpNtlmAuth("domain\\*ä****","*****")
session.get("https://Server2012.domain/RDWeb/FeedLogin/WebFeedLogin.aspx", verify=False)

i get this Error:
Traceback (most recent call last): File "/work/test/ntlmtest.py", line 7, in <module> session.get("https://win2012r2-1.dev.test/RDWeb/FeedLogin/WebFeedLogin.aspx", verify=False) File "/home/IGEL/arlt/.local/lib/python3.5/site-packages/requests/sessions.py", line 521, in get return self.request('GET', url, **kwargs) File "/home/IGEL/arlt/.local/lib/python3.5/site-packages/requests/sessions.py", line 508, in request resp = self.send(prep, **send_kwargs) File "/home/IGEL/arlt/.local/lib/python3.5/site-packages/requests/sessions.py", line 625, in send r = dispatch_hook('response', hooks, r, **kwargs) File "/home/IGEL/arlt/.local/lib/python3.5/site-packages/requests/hooks.py", line 31, in dispatch_hook _hook_data = hook(hook_data, **kwargs) File "/home/IGEL/arlt/.local/lib/python3.5/site-packages/requests_ntlm/requests_ntlm.py", line 151, in response_hook kwargs File "/home/IGEL/arlt/.local/lib/python3.5/site-packages/requests_ntlm/requests_ntlm.py", line 121, in retry_using_http_NTLM_auth server_certificate_hash=server_certificate_hash File "/home/IGEL/arlt/.local/lib/python3.5/site-packages/ntlm_auth/ntlm.py", line 216, in create_authenticate_message msg = self._context.step(self._challenge_token) File "/home/IGEL/arlt/.local/lib/python3.5/site-packages/ntlm_auth/ntlm.py", line 86, in step cbt_data=self.cbt_data File "/home/IGEL/arlt/.local/lib/python3.5/site-packages/ntlm_auth/messages.py", line 338, in __init__ self.user_name = user_name.encode(encoding_value) UnicodeEncodeError: 'ascii' codec can't encode character '\xe4' in position 1: ordinal not in range(128)