To reproduce:

• Go Administration > Websites > Manage
• Click "Add a website"
• Leave the site name empty
• Click "Save"
• It writes out the error "Please specify a value for sitename"

• Then enter a site name
• Expected that the Save button would be green again after the error occurred, so we can try again.
• Got instead: it stays in this state with the Save button greyed out and unclickable:

FYI Marking as regression as I just checked and it was working in 4.X