Currently we only support CredSSP using the Kerberos method (bug 2571). This however requires that the ThinLinc server and WTS both are joined to the same domain, and that all the Kerberos stuff is properly set up on the ThinLinc server.
This is often not the case, and the end result is that ThinLinc cannot connect to a modern WTS by default (as it requires CredSSP by default).
We should add support for NTLM to improve our out-of-box experience with modern Windows servers.
https://fedorahosted.org/gss-ntlmssp/ is a GPLv3 implementation of:
[MS-NLMP]: NT LAN Manager (NTLM) Authentication Protocol
Also note that Windows 2016 supports yet another CredSSP credential type:
rdesktop (and associated tools) is being removed from the ThinLinc product.