I also got this problem using the CSIR IdP. Using the catch-all IDP worked fine as it always has... but this is not a solution for you.
Discussing with @MarcoFargetta, we think that EdX's SP does not do identity disambiguation of the same person from different IdPs, so you end up with the first error you saw. At this point, I'm not sure why the CSIR IdP is giving an external auth problem - there may be an issue with that IdP, or this may be tied to the SP itself.
I will ask @MarioTorrisi_f8dd to check what IdP your user name is associated with on courses.
In the meantime, I think that just adding a space between "Kasandra" and "Isaac" when you sign in with the google IdP may do the trick...