400 Bad Request on HttpWebRequest in VB.Net with Twitter Developer API - vb.net

Question

I just started using the Twitter dev API and I got some issues with it.
Without going into any details about my code, I know the problem lies within the HttpWebRequest:
...
'create WebRequest
Dim request As WebRequest = HttpWebRequest.Create(urlFinal)
request.PreAuthenticate = True
request.Method = URL_METHOD
request.Headers.add("authorization", authString)
request.ContentType = "application/json"
'execute it and open reader
Dim response As WebResponse = request.GetResponse()
Dim reader As StreamReader = New StreamReader(response.GetResponseStream())
'parse WebResponse into String to use it...
Dim rawresp As String
rawresp = reader.ReadToEnd()
...
URL_METHOD is "GET" and I'll explain my authString below. The URL is variable but could be something like https://api.twitter.com/1.1/search/tweets.json?q=abc.
Whenever this is executed, I receive a "400 - Bad Request" error on the line
Dim response As WebResponse = request.GetResponse()
I did some research on it and found that it either is a problem with the URL or with the authentication.
The URL is fine, I can check this quite easily with breakpoints in debug mode. When I open the URL in a browser I receive the error:
{"errors":[{"code":215,"message":"Bad Authentication data."}]}
So it must be the authentication.
Here is how my authString looks like:
Dim authString As String = "OAuth"
authString += " oauth_consumer_key=""" & OAUTH_KEY & """"
authString += ", oauth_nonce=""" & OAUTH_NONCE & """"
authString += ", oauth_signature=""" & OAUTH_SIGN & """"
authString += ", oauth_signature_method=""" & OAUTH_SIGN_METHOD & """"
authString += ", oauth_timestamp=""" & OAUTH_TIMESTAMP & """"
authString += ", oauth_token=""" & OAUTH_TOKEN & """"
authString += ", oauth_version=""" & OAUTH_VERSION & """"
I also checked this during runtime and I get this:
OAuth oauth_consumer_key="xxx", oauth_nonce="xxx", oauth_signature="xxx", oauth_signature_method="HMAC-SHA1", oauth_timestamp="1571760986", oauth_token="xxx", oauth_version="1.0"
OAUTH_TOKEN (Access token) and OAUTH_KEY (API key) come from here:
OAUTH_NONCE (Nonce) and OAUTH_TIMESTAMP (timestamp) are calculated like this:
OAUTH_NONCE = Convert.ToBase64String(New ASCIIEncoding().GetBytes(DateTime.Now.Ticks.ToString()))
OAUTH_TIMESTAMP = Convert.ToInt64((DateTime.UtcNow - New DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc)).TotalSeconds).ToString()
OAUTH_SIGN (signature) like this:
Dim baseFormat = "oauth_consumer_key={0}&oauth_nonce={1}&oauth_signature_method={2}&oauth_timestamp={3}&oauth_token={4}&oauth_version={5}&status={6}"
Dim baseString = String.Format(baseFormat, OAUTH_KEY, OAUTH_NONCE, OAUTH_SIGN_METHOD, OAUTH_TIMESTAMP, OAUTH_TOKEN, OAUTH_VERSION, Uri.EscapeDataString(URL_METHOD))
baseString = String.Concat(URL_METHOD & "&", Uri.EscapeDataString(URL_BASE), "&", Uri.EscapeDataString(baseString))
Dim compositeKey = String.Concat(Uri.EscapeDataString(OAUTH_KEY_SECRET), "&", Uri.EscapeDataString(OAUTH_TOKEN_SECRET))
Using hasher As New HMACSHA1(ASCIIEncoding.ASCII.GetBytes(compositeKey))
OAUTH_SIGN = Convert.ToBase64String(hasher.ComputeHash(ASCIIEncoding.ASCII.GetBytes(baseString)))
End Using
Signature method and OAuth version are hard coded (see above, you might need to scroll to the right).
Can anyone please explain to me where the error is?
I appreciate every answer and am keen to try it all out, I just don't have any other idea anymore.
Thank you very much in advance.
Ben