Like mk24 says, that is a normal "host key changed" error on the PC that you are making the ssh connection from. The ssh client stores fingerprints of the servers that you connect to, and if the fingerprint is different in a new connection, you get this error.
As the host key is automatically generated, you will get this error after you reflash the router (the ssh server) without keeping settings. Or if a new router gets an IP address that belonged earlier to another device, so that the ssh client sees a new host key that differentiates from the one in its database.
The easy fix is to run this command in the PC that you are making connections from:
Too bad that you did not show the actual error message initially...