Inserting h2 and h3 tags turns words into code

Hi,
I am busy optimizing some of my older posts and in the process I want to add h2 and h3 tags to some of the text. I Located the text I want to mark with the above tags in the HTML. I used the tags as follows: <h2>Content text</h2>.
However when I then look at my live posts this code |056707d4fbbe1a8d6525d380edc23a7c| or variants of it is displayed instead of the words.
Am I doing something wrong? How do I fix this?

Thanks.