Unless the server operator was a total dofus, this brings them exactly zero steps towards resolving their problem, because this is exactly the kind of attack that Mixmasters was designed to withstand.
I'm not sure you're right about that. Unlike the more recent Mixminion design, Mixmaster doesn't provide forward secrecy. Each mix uses a long-term public/private key pair. To send a message anonymously, you encrypt it with the public key of each mix you want it to pass through, and each mix uses its own private key to remove a layer of encryption. The last mix in the chain removes the last layer of encryption and delivers the message to its destination. The mixes carry on using the same key pairs indefinitely.
Now imagine you have the wiretapping and server-seizing powers of the FBI and you want to trace a message. You wiretap all the mixes and record the encrypted messages passing between them. When an unencrypted bomb threat pops out of one of the mixes, you seize that mix and use its private key to decrypt all the messages you recorded arriving at that mix. One of them decrypts to the bomb threat. You seize whichever mix that message came from and repeat.
This attack has been known about ten years, which is why Mixminion changes its key pair periodically and uses TLS on the connections between mixes. But remailers don't get much attention these days, so it seems people are still using Mixmaster.
TL;DR: You can trace messages by seizing Mixmaster servers. Expect more servers to be seized in the coming days.