Unexpected token o in JSON at position 1

We receiving this error on console if wpfc is activated. Then some other java-scripts are not loaded. What can we do?

Uncaught SyntaxError: Unexpected token o in JSON at position 1
    at JSON.parse (<anonymous>)
    at Object.set (fgsul.js:69)
    at HTMLDocument.<anonymous> (fgsul.js:719)
    at i (jquery.js:2)
    at Object.fireWith [as resolveWith] (jquery.js:2)
    at Function.ready (jquery.js:2)
    at HTMLDocument.J (jquery.js:2)

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