Full credit to Robert Hedblom for this!
This was 100% taken from his blog and I am just uploading this to ensure that I always have it to refer to as it is an incredibly useful tip.

What is the error?

When you launch Microsoft’s System Center Data Protection Manager (DPM), you receive an error saying “MMC cannot open the file. This may be because the file does not exist, is not an MMC console, or was created by a later version of MMC. This may also be because you do not have sufficient rights to the file.”

What is the cause?

Basically, when you launch the DPM Console it launches a file stored in your profile, inside your AppData. This file becomes corrupt. For me, it was after the DPM 2016 Update 7 Rollup.

How do we fix it?

Easy, delete the file! DPM will automatically recreate the file for you.

Browse to C:\Users\yourusername\AppData\Roaming\Microsoft\MMC and delete the file named Microsoft System Center 2016 Data Protection Manager or whatever it is, depending on your version.