I'll have another look and see if I can replicate the stack error - it's thrown at the same line as the original issue, so I'd assumed the two were related.

Will get a patch release out soon.

Did the "Maximum call stack size exceeded" origin become known? I just upgraded to Umbraco 7.15.3 and am getting this error. Fixed it by replacing 3 instances of _typeof(something) with typeof(something) - removing the underscore. Seems like typeof is getting replaced by _typeof somehow and causing a self-reference. line 12 of workflow.js [corrected]:

function _typeof(obj) { 
if (typeof Symbol === "function" && typeof(Symbol.iterator) === "symbol") { 
    _typeof = function (_typeof2) { 
        function _typeof(_x) { 
            return _typeof2.apply(this, arguments);
        } 
        _typeof.toString = function () { 
            return _typeof2.toString();
        };
        return _typeof;
    }
    (function (obj) { 
        return typeof obj === "undefined" ? "undefined" : typeof(obj); 
    }); 
} else { 
    _typeof = function (_typeof3) { 
        function _typeof(_x2) { 
            return _typeof3.apply(this, arguments); 
        }
        _typeof.toString = function () {
            return _typeof3.toString(); 
        };
        return _typeof;
    }
    (function (obj) {
        return obj && typeof Symbol === "function" && obj.constructor === Symbol && obj !== Symbol.prototype ? "symbol" : typeof obj === "undefined" ? "undefined" : typeof(obj); 
    }); 
}
return _typeof(obj); 

}

typeof(Symbol.iterator)

and two instances of

typeof obj === "undefined" ? "undefined" : typeof(obj); 

Could Umbraco be using babel/reactjs and be causing this? https://stackoverflow.com/questions/35392666/babel-requirejs-typeof-rangeerror-maximum-call-stack-size-exceeded

That's all compiled JavaScript - I have a transpiling step in the build to target es5, so possibly something is getting mixed up in that process. Updating anything in the workflow.js file won't be conclusive.

It's all open source of you feel like pulling down the source and finding the original source of the problem.

Are you using the latesr plumber release?

plumber - v1.1.12-build402 - 2020-01-24