Access denied when sending text from RichEditBox through HttpClient POST

Question

I'm using HttpClient to POST a user created string to a server API and get the result. Previously I used a TextBox to allow the user to enter the string but I wanted pretty colours so I tried to replace the TextBox with a RichEditBox but failed miserably. When using the TextBox everything worked fine but when using the RichEditBox I get an "Access denied" message. I get the text from the RichEditBox with the Document.GetText method. I can get it to work by either inserting a static string in the place where I get the text or in the place where I send the string into a FormUrlEncodedContent. The string is edited before and after adding the text from the RichEditBox, and sent to another method.

TL:DR: Sending a string from a TextBox with a HttpClient using POST works but not when replacing the TextBox with a RichEditBox.

Here's the full error message:

System.UnauthorizedAccessException: 'Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED))'

Does anyone have a solution or an explanation to why this is happening?

Edit

Code sample:

private async void RichEditBox_KeyUp(object sender, KeyRoutedEventArgs e)
{
    if (e.Key == VirtualKey.Enter)
    {
        await RunCode();    //The error points to this line
    }
}

private async void RunCode()
{
    string code = CodeBefore;
    foreach (RichEditBox tb in editStack.Children)  //If I comment out the foreach loop I don't get any errors
    {
        if (tb.Tag.ToString() == "input")
        {
            tb.Document.GetText(Windows.UI.Text.TextGetOptions.None, out string thisLine);
            code += thisLine;
        }
    }
    code += CodeAfter;
    await RunCSharp(code);
}

private async Task<Code> RunCSharp(string code)
{
    Code re = new Code();

    using (HttpClient client = new HttpClient())
    {
        FormUrlEncodedContent content = new FormUrlEncodedContent(new[]
        {
                new KeyValuePair<string, string>("LanguageChoiceWrapper", "1"),
                new KeyValuePair<string, string>("Program", code),              //If I replace code with a string  I don't get any errors
                new KeyValuePair<string, string>("ShowWarnings", "false")
            });

        try
        {
            HttpResponseMessage msg = await client.PostAsync("http://mywebaddress.com/run", content);
            re = JsonConvert.DeserializeObject<Code>(await msg.Content.ReadAsStringAsync());
        }
        catch { }
    }

    return re;
}