Error message when I edt

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Hello all,
Since upgrading to 5.6 when I do an edit I get the following message
{"aID":"86","arHandle":"Main","cID":"128","error":false,"bID":"105"}#next_pages_container { width: 5px; hight: 5px; position: absolute; top: -100px; left: -100px; z-index: 2147483647 !important; }
Then it appears that nothing happens until you leave edit mode. Is this normal?
Sorry if this is a stupid question, I'm very new to this, so there'll probably be many more.
Thanks, Straz.