The error looks not related to number of threads running in the datanode. It really looks related to connection problem.
It would be really helpful if you can provide more detaile stacktrace.
My GUESS is that there could be chances that, Next datanode in the pipeline (given by the namenode) is down. So first datanode is not able to connect to next datanode and throwing the above mentioned exception. Since you have 6 datanodes, writes could be successful with remaining nodes in the cluster.