This looks like one for @MarioTorrisi_f8dd who has access to the logs of the EdX server. @MarioTorrisi_f8dd can you check what the error is with this particular user ?
@SakhiHadebe_1c36 can you please confirm that this user has indeed refreshed their cache and deleted cookies ?
Is this problem happening on any of the other services (ie, discourse, science gateway, etc) ?
Unfortunately, I can't reproduce this problem, because I'm already registered - using SAFIRE, I get
External Authentication failed
You have already created an account using an external login like WebAuth or Shibboleth.
Please contact firstname.lastname@example.org for support
There is no identity merging by the service.
@SakhiHadebe_1c36 have you checked