Hi David,

Forgot to mention that the textstring comes from a custom property editor. I forgot to implement a Property Value Converter. That was the reason. Now everything works fine. Here's the documentation (https://our.umbraco.com/documentation/Extending/Property-Editors/value-converters) and my implementation below:

using System;
using Umbraco.Core;
using Umbraco.Core.PropertyEditors;
using Umbraco.Core.Models.PublishedContent;
using Umbraco.Web;

namespace Plugins.MyTextbox
{
    [PropertyValueType(typeof(String))]
    [PropertyValueCache(PropertyCacheValue.All, PropertyCacheLevel.Content)]
    public class MyTextboxPropertyConverter : IPropertyValueConverter
    {
        public object ConvertDataToSource(PublishedPropertyType propertyType, object source, bool preview)
        {
            return Convert.ToString(source);
        }

        public object ConvertSourceToObject(PublishedPropertyType propertyType, object source, bool preview)
        {  
            return source;
        }

        public object ConvertSourceToXPath(PublishedPropertyType propertyType, object source, bool preview)
        {
            return source.ToString();
        }

        public bool IsConverter(PublishedPropertyType propertyType)
        {
            return propertyType.PropertyEditorAlias.Equals("MyCustom.MyTextbox");
        }
    }
}