Redirect to front page on event registration and mailing list registration in Joomla 2.5.11

A clients website is returning a variation in experience when users try and register for events and also when they try and subscribe to the mailing list (which is a contribution page).

On my computer i have never had the issue but several users (seems random) report being unable to do either (above) as they get redirected to the front page every time they click 'contribute'.

Joomla version is 2.5.11 and civiCRM is 4.3.3. Users report issue using several different browsers