Are you really using the square brackets? I suspect, if you are, that somehow the code has been corrupted at that point during the conversion.
Create a new blank layout and see if you can enter the text. If so... I suspect that the layout or something on it has corrupted. Make a new one...
FM 12 has CSS and may have misinterpreted something during conversion. (eg. mistaken text for code or only partially converting an external image or such...
I have done the same and gradually getting out of the mess, since I have
dozens of layouts. Where ever there is corrupt objects, I am deleting it
and creating new layouts in FM12