To reproduce:

• create a NODE
• goto configure NODE
• from inside the configuration page of the node select SSH node
• from inside the configuration page of the node select "Add Credentials"
• from the Credentials form view, select SSH-key type and "insert directly"
• try to insert a key
• it is impossible to scroll down to "save" or "cancel" the configuration

The workaround:

generate Credentials to be used from the Slave from the main "Credentials" section in the left panel of Jenkins