I try to add a global credential for Jenkins and get an 'Oops' page.

specifically:
Jenkins -> Credentials -> Global credentials (unrestricted)

Next:
Add credentials

In that form I choose "SSH Username with private key".
Then select the 'OK' button.

Next:
Then select that new 'jenkins' credential.
Then 'Update'.
Then select "From the Jenkins master ~/.ssh". It is a passwordless id_rsa private key file.
In the form I choose "SSH Username with private key"

Boom. See attached oops.txt file.