(copied from my answer on meta.so)
The servers where the errors happened just got patched. The running theory is that that in some cases, when app domains that existed before that patching tried to load new assemblies after the patching, there was some conflict that prevented that from working.
Exception type: FileLoadException
Exception message: Loading this assembly would produce a different grant set
from other instances. (Exception from HRESULT: 0x80131401)
This is so low-level that you didn't even get our custom error page if you hit an affected server (which wasn't all of them, so not everybody saw any issues).
We'll look deeper into the causes later, but the issue is fixed for now and it's late at night for our SREs in the U.S.