SCP problems



I have a strange problem since around three days.
I have a C1 instance with Ubuntu 16.04 installed.
OS is installed on device /dev/nbd0. I have a second volume mounted from /dev/nbd1.
I have a file on nbd1 and try to copy it via scp to my local machine. After a few seconds I get a connection reset error and the scp process exits with failure.
I copy the file internally to nbd0 and start the scp again. The file is transferred successfully.
I create a new file on nbd0, I can scp it to my local machine.
I copy it to nbd1, scp fails.

I tried to archive the instance and recreate it.
I cloned the volume to a new one and attached the new one to the server.
Nothing helped. The problem persists, that scp from nbd1 to local machine fails.

To exclude my local machine as an source of failure, I tried the same scp commands from another remote machine. Same problem.

Maybe someone has seen this problem already or maybe has an idea what this could be.