If a form contains a Checkboxes form widget and it is configured to be required, when submitting the form on the frontend "Entry is not valid!" error will be thrown if more than one options are selected in this Checkboxes form field.

For more information about Bug 199652, see http://feedback.telerik.com/Project/153/Feedback/Details/199652

Fixed in Sitefinity 10.1.6500.0.

Extend the CheckboxesFieldModel and override the IsValid() method to apply custom logic for validating the field.

Perform the below steps in order to extend the model:

1. Open the project in Visual Studio
2. Create a class that inherits from CheckboxesFieldModel
3. Override the IsValid() method and validate the checkbox field in this method.

Here is a sample implementation:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using Telerik.Sitefinity.Frontend.Forms.Mvc.Models.Fields.CheckboxesField;

namespace SitefinityWebApp
{
    public class CheckboxesFieldModelCustom : CheckboxesFieldModel
    {
        public override bool IsValid(object value)
        {
            if (this.ValidatorDefinition.Required.HasValue && this.ValidatorDefinition.Required.Value && !this.HasOtherChoice)
            {
                var strValue = value as string;

                if (!string.IsNullOrEmpty(strValue))
                {
                    // split multiple selected values: e.g. Choice1,Choice2
                    var inputValues = strValue.Split(',');

                    var choices = this.DeserializeChoices();

                    // if the choices array contains all the input values, return isValid
                    if (choices.Intersect(inputValues).Count() == inputValues.Length)
                    {
                        bool baseIsValid = this.Validator.IsValid(value);
                        return baseIsValid;
                    }
                }

                return false;
            }
            else
            {
                return this.Validator.IsValid(value);
            }
        }
    }
}

4. Register the custom model in the Global.asax file as shown below:

using System;
using Telerik.Sitefinity.Abstractions;
using Telerik.Sitefinity.Frontend;
using Telerik.Sitefinity.Frontend.Forms.Mvc.Models.Fields.CheckboxesField;

namespace SitefinityWebApp
{
    public class Global : System.Web.HttpApplication
    {

        protected void Application_Start(object sender, EventArgs e)
        {
            Bootstrapper.Initialized += new EventHandler<Telerik.Sitefinity.Data.ExecutedEventArgs>(this.Bootstrapper_Initialized);
        }

        protected void Bootstrapper_Initialized(object sender, Telerik.Sitefinity.Data.ExecutedEventArgs e)
        {
            if (e.CommandName == "Bootstrapped")
            {
                FrontendModule.Current.DependencyResolver.Rebind<ICheckboxesFieldModel>().To<CheckboxesFieldModelCustom>();
            }
        }

    }
}

5. Build the solution
6. Restart the application

There is a similar error message reported in version 14.4 when adding comma character (,) to the options. This error is currently being fixed.
Keep tuned with the upcoming patches.