DPM 2012 R2 is backing up Hyper-V 2012 R2 in offline state

I upgraded our Hyper-V 2008 R2 failover cluster to Hyper-V 2012 R2, but since then some of the VMs are not able to be backed up in online state.

If you look at the event log on the Hyper-V host you will straight away find your answer there. 

As you can see from the error message above, it requires you to upgrade your Integration Services on your VM. You will have to restart your VM after the upgrade. 

Once you have done it, you will be able to backup your VM in online state