Hello Kacper,
did you solve this problem, if so how? We have same problem.
Mikrotik RB1100AHx4 ROS 6.41.4
Debian 9:
FreeRADIUS Version 3.0.12
Version 4.5.12-Debian
Windows 10 build 1709
SSTP client same error.
Thank you.
Ragards
Ondrej