A sign-in with another subject of the same provider is refused, so an
account whose provider re-issued the subject (a migrated or rebuilt IdP, a
changed sub format) was locked out: editing the email, as the wiki advised,
never touched the stored link. The edit dialog now offers "Remove SSO link"
for other accounts while SSO is set up (DELETE /api/admin/users/:id/sso,
audit admin.user_sso_unlink). The next SSO sign-in links again under the
rules of a first one, so it still needs a verified, not self-set email.
The log line names the remedy, and the wiki tells the two refusals apart.
Fixes#3065
Maurice
b53130ca0d