Comments bug?

I have built a training app which includes the feature Comments, but there is a functionality problem I cannot resolve:

I cannot leave a comment using my smartphone, only from a computer. On both platforms I am told to enter my name and email together with a pin code sent by mail. When using my smartphone the pin code never arrives in the mail, but when using my computer it does.

Friends who use the training app on their smartphones and try to leave comments never receive an email with pin code either, so it is not only my own smartphone. Is there anything I can do to “reset” all registered users of comments in case something has “locked up”?