Can't start ec2 instance after re-attaching ebs volume

I had to extend the EBS storage of my VPC Windows Server 2012 instance. I did this following the official documentation http://ift.tt/1DmCVtQ. I created first a snapshot, then a volume, detached the old volume, attached then the new, all through the standard AWS UI. When I'm trying to start the instance an error message pops up: Error starting instances (...) Instance does not have a volume attached at root /dev/sda1.

Any ideas?

Thanks

PS

I'm aware of this post EC2 Instance doesn't start after Reattaching the Volume however trying the steps mentioned in there doesn't solve my problem. I created a new EC2(VPC) instance; after creation it needs first to be launched. After that de-taching the new volume and re-attaching the (old) extended ebs volume that has been created from the snapshot brings up the same error message above.