This problem can occur when the RSA keys are corrupt.
In order to resolve this issue, zeroize the RSA keys and do not create the key again with the crypto key generate command, but re-connect Internet Explorer using HTTPS.
The router should enable Secure Shell (SSH) and generate an SSH 768 key as soon as the browser client connects.
Issue the crypto key zeroize rsa command in order to zeroize the RSA keys in the configuration.
Note: It is not recommended to use both HTTP and HTTPS for SDM access.
Issue the no ip http server command in order to remove the HTTP access.