URLs not recognized as valid during SAML IdP app creation


I am trying to configure Okta as Identity Provider (IdP) for a SAML authentication flow. The Service Provider (SP) is our own application. However, when trying to use our URLs as Single sign-on URL or Single logout URL, I keep getting the error msg “Is not a valid URL. Valid example: http://www.okta.com”. Examples for the URLs used:

The protocol (http vs. https) makes no difference. Neither if I add “www” or not.
Why is that and what URL format does Okta expect here?
Thanks a lot.