A.7.14 Why do I see ‘Couldn't load private key from ...’? Why can PuTTYgen load my key but not PuTTY?

It's likely that you've generated an SSH protocol 2 key with PuTTYgen, but you're trying to use it in an SSH-1 connection. SSH-1 and SSH-2 keys have different formats, and (at least in 0.52) PuTTY's reporting of a key in the wrong format isn't optimal.

To connect using SSH-2 to a server that supports both versions, you need to change the configuration from the default (see question A.2.1).