TL;DR: The CryptohomeInterfaceProxy class uses "/org/chromium/" as object path, whereas Cryptohome uses "/org/chromium/Cryptohome". This makes the proxy crash whenever any method is called.

In org.chromium.CryptohomeInterface.xml, we set
  <node name="/org/chromium/">
This node name is used by platform2/chromeos-dbus-bindings/proxy_generator.cc to write the object_path_ variable for the generated cryptohome-client/cryptohome/dbus-proxies.h file. object_path_ is then passed into 
bus_->GetObjectProxy(service_name_, object_path_)}

This path is wrong. platform2/cryptohome/cryptohome.cc actually uses cryptohome::kCryptohomeServicePath as object path / service path.