Time Machine backup error

I'm running a Mac Mini i5 mid 2011 and I bought a WD My Cloud 3TB to back up and use files remotely with multiple users. I connected the cloud to the Mac and performed my first Time Machine backup of 350gb of data and after a couple of days it was complete. Over the next day or so there were no problems, but yesterday I had to unplug the cloud drive to reposition it. Since then I have not been able to back up and get this message.

The operation couldn’t be completed. (OSStatus error 2.)

I can still access the drive in Finder and can still access it via my iPad and iPhone but I can't back up, which is hugely frustrating.

Posted here as I am a Mac novice and this is my first experience of Time Machine.

Any help greatly appreciated.