Recently saw few cases when the Azure AD SP initiated SAML SSO is returning the error – “This login.onmicrosoft.online.com page can’t be found. No webpage was found for the web address”.
If you pay closer attention, you can notice the HTML encoded space “%20” symbol right after Azure AD SAML Login URL string.
Checking the application federation SSO settings revealed the space really being present in the IdP Login URL. After removing it, the mentioned error was resolved.
You can easily reproduce the issue by registering Microsoft SAML Toolkit in your tenant and adding the space in the Login URL field.
UPDATE: As of end of August 2020 this issue should have been addressed and Azure AD should be able to strip the space automatically from the end of Azure AD SAML Login URL string.