We have serious issues with ES 2.1. and Marvel.
Cluster is becoming Yellow and all the writes operations were blocked ! In state, it's not normal. Everything came back to normal when we removed Marvel plugin.
Here is the log: https://gist.github.com/anonymous/930ff95e9172358fe81c
The log indicates that a node left and so marvel failed to retrieve statistics for the shards (series_556) that was located on this node. It makes sense and I don't think that this is not what blocked the writes operations.
Can you check if you have other error messages in your logs? Can also you give us the output of
GET /_cat/shards?v ?