Could you add this line into your web.config

<clientResources debug="true" />

It will make Forms load un-minified EPiServerForms.js so you can debug to see what the error is. You need to have css classes [Form__Element Form__CustomElement] or data attribute [data-epiforms-element-name] in your element markup file. And also you need override the method getCustomElementValue to return your custom element value.

Hope this help

Thanks that helps it's actually the validatorActor that is the problem, it's not set (i.e. null).  I'm not sure where the validatorActor comes from but there should probably be a validation check of some sort in the JS if it is required.

 function validateFormValue(fieldName, fieldValue, validators) {
                var results = [];

                $(validators).each(function (index, item) {

                    // take the Actor, it can be function to execute, or contain property (of type function) "validate"
                    var validatorActor = epi.EPiServer.Forms.Validators[item.type];
                    var validatorFunc = null;

                    if (typeof validatorActor === "function") {
                        validatorFunc = validatorActor;
                    } else if (typeof validatorActor["validate"] === "function") {
                        validatorFunc = validatorActor["validate"];
                    }

                    // execute the validatorFunc
                    if (validatorFunc) {
                        var itemResult = validatorFunc(fieldName, fieldValue, item);
                        $.extend(itemResult, { fieldName: fieldName, fieldValue: fieldValue });
                        results.push(itemResult);
                    }
                });

                return results;
            }

You need extend epi.EPiServer.Forms.Validator to add your own for the element. Please see the EPiServerFormsSample.js (line 127)

// extend the EpiForm JavaScript API in ViewMode
$.extend(true, epi.EPiServer.Forms, {
    /// extend the Validator to validate Visitor's value in Clientside.
    /// Serverside's Fullname of the Validator instance is used as object key (Case-sensitve) to lookup the Clientside validate function.        
    Validators: {
        "EPiServer.Forms.Samples.Implementation.Validation.AddressValidator": addressesValidate,
        "EPiServer.Forms.Samples.Implementation.Validation.DateTimeRangeValidator": dateTimeRangeValidate,
        "EPiServer.Forms.Samples.Implementation.Validation.DateTimeValidator": dateTimeValidate,
        "EPiServer.Forms.Samples.Implementation.Validation.DateValidator": dateTimeValidate,
        "EPiServer.Forms.Samples.Implementation.Validation.TimeValidator": dateTimeValidate,
        "EPiServer.Forms.Samples.Implementation.Validation.RecaptchaValidator": function (fieldName, fieldValue, validatorMetaData) {
            // validate recaptcha element
            if (fieldValue) {
                return { isValid: true };
            }

            return { isValid: false, message: validatorMetaData.model.message };
        }
    },
......

Yeah that worked thanks.  This really needs to be included in the documentation for EPiServer Forms.

Also how does the loading of scripts for EPiServer Forms work with the current implementation for EPiServer (EPiServer.Framework.Web.Resources.ClientResources).  If  EPiSErver Forms works differently this is going to be really annoying.  I would have expected that it would use the existing client resource framework but it doesn't seem to use this.