SQL CLR randomly fails in SQL Server

I have created a .Net assembly and imported it into SQL Server and use it in a scalar-valued function (SVF).

When I execute the SVF it works fine. The second time also. But every third time the SVF fails and the underlying error is

System.NullReferenceException: Object reference not set to an instance of an object

I have added try/catch statements and debug data to the CLR, but I still have no clue as to why this CLR fails.

Can you help me out?

This is the C# code:

[Microsoft.SqlServer.Server.SqlFunction(DataAccess = DataAccessKind.Read)]
public static SqlString HEAD(SqlString Uri)
{
    string statusCode = "";
    // Debug info
    string debugData = "1,";

    try
    {
        debugData += "2,";
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
        debugData += "3,";
        HttpWebRequest req = (HttpWebRequest)WebRequest.Create(Convert.ToString(Uri));

        req.Method = "HEAD";
        req.AllowAutoRedirect = true;
        debugData += "4,";
        HttpWebResponse resp = (HttpWebResponse)req.GetResponse();
        debugData += "5,";

        if (resp.StatusCode == HttpStatusCode.MethodNotAllowed)
        {
            // Use GET instead of HEAD
            debugData += "6,";
            req = (HttpWebRequest)WebRequest.Create(Convert.ToString(Uri));
            req.Method = "GET";
            resp = (HttpWebResponse)req.GetResponse();
        }

        debugData += "7,";
        statusCode = ((int)resp.StatusCode).ToString();
        debugData += "8,";
    }
    catch (WebException ex)
    {
        debugData += "9,";
        HttpWebResponse webResponse = (HttpWebResponse)ex.Response;
        debugData += "10,";
        statusCode = ((int)webResponse.StatusCode).ToString();
        debugData += "11,";
    }
    catch (Exception ex)
    {
        debugData += "12,";
        statusCode = debugData + ex.Message;
        debugData += "13,";
    }
    debugData += "14,";
    //return (statusCode);
    return debugData;
}

The method takes a string as input (which is a URL) and checks if the URL actually exists by doing a HttpWebrequest with a HEAD method.

It returns the status code, or an error message (hence the string as output).

The assembly has been successfully imported into my database:

ALTER DATABASE MyDatabase SET TRUSTWORTHY ON;
GO

CREATE ASSEMBLY SQLHttpRequest
FROM N'C:\somefolder\SQLHttpRequest.dll'
WITH PERMISSION_SET=UNSAFE;
GO

And this is the SVF:

CREATE FUNCTION dbo.uf_head_webrequest
    (@Uri nvarchar(max))
RETURNS nvarchar(max)
AS
EXTERNAL NAME SQLHttpRequest.[SQLHttpRequest.HTTPFunctions].HEAD;

This is how I test this function:

declare @uri nvarchar(1000) = 'https://logos-download.com/wp-content/uploads/2016/02/Microsoft_logo_SQL-700x566.png'
SELECT dbo.uf_head_webrequest(@uri)

The output is 1,2,3,4,5,7,8,14,

However, if I execute it a couple of times, I get this response:

Msg 6522, Level 16, State 1, Line 2
A .NET Framework error occurred during the execution of user-defined routine or aggregate "uf_head_webrequest":
System.NullReferenceException: Object reference not set to an instance of an object.
System.NullReferenceException:
at SQLHttpRequest.HTTPFunctions.HEAD(SqlString Uri)
.

I understand the concept of object instantiation but don't understand why I don't get any debug data back.

I have searched the internet and came up with this question on SO:

SQLCLR .NET Error: Object reference not set to an instance of an object

This describes problems with NULL values. But in my opinion, this is not the case here. Especially because I am using the same Uri again and again.

Thanks!