Serious problem with MySejahtera registration

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I would like to highlight a problem with the MySejahtera app that nobody seems to have raised so far: There seems to be no way at all to get any help if there is an issue with the app and/or a user ID being deemed invalid.

I attempted to register my 74-year-old father's mobile number and IC in the app in late February to ensure he will be in line for the government’s Covid-19 vaccine. I followed the steps as required, got the OTP (one time password) through his mobile and appeared to have completed the registration process. However, when I tried to sign in to complete the vaccination registration, the app deemed his mobile number an "Invalid ID”. I went through the process several times and each time got the same message about an invalid ID.