Hi Dan,
I was asked to politely let you know that there's a chance it may not be the same issue as the one you linked; and to point out that our specific issue had to do with the password-change endpoint in particular (it was returning a 500 Server Error, even in the official FusionAuth admin panel). We are also on a higher version than the other gentleman.
However the issue described in the other post also suggests that the change-password endpoint is failing to work as expected, which may indicate a bug in the code, and so you may be spot on in thinking that the issues are related.