Ubuntu – Ubuntu 16.04((HyperV)-very large file in /tmp,keeps growing at about 1GB/h-large CPU and network load while system is idle

I'm ICT coördinator at a high school and I'm running a Plex server on an Ubuntu 16.04 machine in VMware. This one runs flawlessly.

However, we're moving our servers to a data center, so I'm setting up a Plex server there. I have installed everything on a HyperV machine in the data center. This is my set-up:

• 4 CPU's
• 1TB of storage
  

I'm testing the server and I noticed some strange things, while I'm doing nothing at the Ubuntu Machine, only accessing it via xrdp/RDP:

• There's a file in /tmp that grows at a rate of about 10GB per hour. I
  have no idea which process is responsible for this. (it's the big one: dup-e7e7…)
  
• KSysGuard shows an almost constant CPU usage of over 30%
• KSysGuard also shows a constant stream of incoming network traffic at a rate of about 5MiB/s
  

I'm currently running a identical Plex server on a VmWare setup and I don't have any of these problems there, this one has been running flawlessly for over 4 months now.

Any thoughts on what could be the problem or on how I could find what's causing this?

I've tried lsof, auditctl, no results…

Thanks in advance!