I have no clue what is going on, but it is a taleworlds problem, since every mp mod is affected.
In the official forum they have posted a quick fix.
search for the rgl_config.txt in the "c:\Users\Username\Documents\Mount&Blade Warband" folder and change the
"use_secure_connection = 1" to "use_secure_connection = 0" and save the .txt file (sometimes i had to save twice).
This has worked for me.