This seems odd.
To my memory, Silverstag actually goes the extra mile and installs WSE along with the mod files, including a compatible executable from the version of MnB that WSE can use.
Could you check your MnB installation folder and check if you have a "mb_warband.exe" AND a "mb_warband_old.exe"?
If I had to guess, you either aren't using WSELoader.exe to launch, or tried to install WSE yourself in addition to Silverstag (Silverstag - again if I remember right - sets WSE up to launch "mb_warband_old.exe", which is the compatible executable, so Steam still launches the new one. But a fresh install of WSE on top of Silverstag could make WSE try to use mb_warband.exe, which is not compatible.)