Error Message when trying to publish edits to local blocks

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When I try and edit a local block (specific to a page type), I keep getting the following error message after clicking update:

{"aID":"65","arHandle":"FP Services","cID":"1","isGlobalBlock":false,"error":false,"bID":"76"}


This happens the first time I try to edit, so I click cancel and try to edit the block again and then I get 'access denied'.

I'm stumped as to what it could be? The site was working fine yesterday.

Many thanks in advance...