BadImageFormatException: Failed to resolve field token

Question

Everything was compiled and worked perfectly until today. As far as I know, nothing has changed in my system.

Suddenly, everything I compile with xbuild (mono 4.0.0) has this runtime error:

[ERROR] FATAL UNHANDLED EXCEPTION: System.BadImageFormatException: Could not resolve field token 0x040000de

I compile and run on x64 under Docker.

Any idea how to debug this?