FileSystemWatcher is only used in the server, for control_dir_v2 which is supposed to prevent multiple instances running.
BadImageFormatException happens when something with the DLLs is wrong. In this case, it looks like some of the system libraries are broken (which is a mono issue).
Different error, but note the object_op and similar stack trace:
The filesystem watcher seems to be a part of mono that breaks sometimes. In that case it was solved by downgrading mono. What version do you have?