Thanks @Schuby for the report,
1) When the existing user switched to the SSO sign-in under Account Settings:
a) were they redirected to the sign in page (sounds like they were)
b) did they receive an email titled "You updated your sign-in method on [server name]"
2) Can you help go to System Console > Teams and
- click on the "+" icon next to "TEAMS" on left-hand sidebar
- in the "Select Team" modal, choose the team users who are having login issues belong to
- go to the "Users" section of the team you just chose on the left-hand sidebar
- search for the user(s) and check what their "Sign-in Method" is on the page
1) and 2) is to confirm their sign-in method was changed successfully.
Hopefully we can get this resolved for you quickly.