open MPI - ring_c on multiple hosts fails

I have recently installed open MPI on two Ubuntu 14.04 hosts and I am now testing its functionality with the two provided test functions hello_c and ring_c. The hosts are called 'hermes' and 'zeus' and they both have the user 'mpiuser' to log in non-interactively (via ssh-agent).

The functions mpirun hello_c and mpirun --host hermes,zeus hello_c both work properly.

Calling the function mpirun --host zeus ring_c locally also works. Output for both hermes and zeus:

mpiuser@zeus:/opt/openmpi-1.6.5/examples$ mpirun --host zeus ring_c
Process 0 sending 10 to 0, tag 201 (1 processes in ring)
Process 0 sent to 0
Process 0 decremented value: 9
Process 0 decremented value: 8
Process 0 decremented value: 7
Process 0 decremented value: 6
Process 0 decremented value: 5
Process 0 decremented value: 4
Process 0 decremented value: 3
Process 0 decremented value: 2
Process 0 decremented value: 1
Process 0 decremented value: 0
Process 0 exiting

But calling the function mpirun --host zeus,hermes ring_c fails and gives following output:

mpiuser@zeus:/opt/openmpi-1.6.5/examples$ mpirun --host hermes,zeus ring_c
Process 0 sending 10 to 1, tag 201 (2 processes in ring)
[zeus:2930] *** An error occurred in MPI_Recv
[zeus:2930] *** on communicator MPI_COMM_WORLD
[zeus:2930] *** MPI_ERR_TRUNCATE: message truncated
[zeus:2930] *** MPI_ERRORS_ARE_FATAL: your MPI job will now abort
Process 0 sent to 1
--------------------------------------------------------------------------
mpirun has exited due to process rank 1 with PID 2930 on
node zeus exiting improperly. There are two reasons this could occur:

1. this process did not call "init" before exiting, but others in
the job did. This can cause a job to hang indefinitely while it waits
for all processes to call "init". By rule, if one process calls "init",
then ALL processes must call "init" prior to termination.

2. this process called "init", but exited without calling "finalize".
By rule, all processes that call "init" MUST call "finalize" prior to
exiting or it will be considered an "abnormal termination"

This may have caused other processes in the application to be
terminated by signals sent by mpirun (as reported here).
--------------------------------------------------------------------------

I haven't found any documentation on how to solve such a problem and I don't have a clue where to look for the mistake on the basis of the error output. How can I fix this?