the form might be being rendered wrong on the web browser. an invisible html layer (or DIV) might be the reason why you can't click on the text box. it's not a solution, but you can try to use a browser plugin, one like DOM Inspector, which explains the HTML object that you are trying to click on to see if this is the case. maybe this helps finding out if this is related to some field in the form (check object IDs).
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~soba