Do you know with what tool the key was generated?
Although very rare, some utilities can produce improperly formatted headers within the key file that can cause issues with importing the key. It is usually related to incorrect carriage returns that some key generators produce that are not to standards.
If you'd like further assistance with this, we will need to take a look at your key and see if we can pinpoint the problem. Alternately, you can see if you are able adjust settings on the tool used to export that may clean it up.
Thanks - Jon