Error while rendering template [htmlheader.vm] when clicking on 'Forgot your username' and 'Forgot your password' making the templates unreachable

but it's strange to me that a criterion like this could change depending if the instance is an upgrade or a fresh one.

I'm not sure that this is really the condition. It might a completely different difference, it's just that so far I only saw the issue on fresh instances and never on an updated one but I didn't actually check on the same type of instances with and without upgrade.