Hi
All systems have been restored and should be functioning normally again.
We are now using Debian Jessie on japsand with the 4.3 kernel coming from jessie-backports. This allows using the in-kernel r8169 driver for the NIC, dumping the old Realtek blob. So far, network connectivity has been stable and the link constantly saturated while downloading a large file from my server with the in-kernel driver.
Additionally, the new hardware seems to have fixed the constant I/O problems. With that, the triggered builds should finish much faster than previously.
I would like to extend my gratitude to our ISP, Hetzner, that acted very fast on our request.
Mihai