The page cannot be displayed because the HTTP version is not supported (306994)

After enabling S2FA for Administrators on PW Manager, login with admin credentials passes but sending either a Push Notification, SMS or Phone Call request fails with one or more of the following errors

“User was not valid”

“The page cannot be displayed because the HTTP version is not supported”

“System error occurred. For more information, view the event log.”

And the following error is reported in the event log

“User was not valid at Starling.TwoFactor.Client.Starling2FAClient.SendSMS(String displayName, String mail, String phoneNumber, String locale, Boolean ignoreIfAppInstalled)”

The format of the users mobile number in Active Directory is incorrect.

For Starling 2FA phone numbers must use E.164 phone number formatting.

Ref: https://support.twilio.com/hc/en-us/articles/223183008-Formatting-International-Phone-Numbers

So phone numbers should have the following in this sequence

1. A + (plus) sign
2. International Country Calling code
3. Local Area code
4. Local Phone number

It should be + 353 87 1234567

and NOT 00 353 87 1234567