Hi there – thanks for the report.
I’ve had a look and I see the problem on your site but it’s not happening on my test site so something must be different between the two.
Firstly – are you sure it’s my plugin? No doubt you’ve checked already but I want to make sure the problem still happens when custom login is disabled.
Secondly – are you perhaps using a different language on your site? I wonder if the translation might be missing.
As far as I know nothing custom login does should interfere with the error messages in the login form so I don’t think it should be the culprit here – but perhaps there’s a conflict with something else that is causing this when custom login is enabled.