Error - Confirm Action

Hi,

Recently i have been approached by one of my forum's member and she said she is having some difficulties in registering there.
upon guiding her on the registration process, found out she is stuck here with the error "Confirm Action"
I remember i used to have this problem when i left the browser / the page ideal for certain of moment and all i need to do is to click on "confirm" button and the post will be posted anyway. but in her case, nope, it is not.

When i asked her to click on "confirm" the page just refreshed itself at the very same page.
I have tried to test it from my end with

- new registration and found no problem
- other members dont have such problem too
- cleared cache, cookies <from her side>
- tried different browser <from her side>
- even tested with iphone, netbook, notebook <from her side>
- tried different range of email addresses (domain name)
- ip address / username  is not been blocked / banlisted.

In such case, if there is any other way we can fix this issue or is it a well known bug ?
Please assists

thank you