When trying to create a new SSH key credential (either one using ~/.ssh or manually inserting in a private key), clicking Add results in a 501 error:

501 /descriptor/com.cloudbees.plugins.credentials.CredentialsSelectHelper/addCredentials

java.lang.NoClassDefFoundError: hudson/util/FormApply
at com.cloudbees.plugins.credentials.CredentialsSelectHelper.doAddCredentials(CredentialsSelectHelper.java:101)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:622)
...

This is a brand new installation of Jenkins. Using Jenkins ver. 1.424.6, Credentials Plugin 1.10, SSH Credentials Plugin 1.6.1, Jenkins GIT client plugin 1.6.2, SCM API Plugin 0.2, Jenkins GIT plugin 2.0.1.

However, the credentials are saved to the system and I can use them in projects.