diff --git a/Duplicati/Library/Backend/Backblaze/B2.cs b/Duplicati/Library/Backend/Backblaze/B2.cs
index a819b10c9..8447b7ed5 100644
--- a/Duplicati/Library/Backend/Backblaze/B2.cs
+++ b/Duplicati/Library/Backend/Backblaze/B2.cs
@@ -322,7 +322,7 @@ public class B2 : IStreamingBackend
             using var request = new HttpRequestMessage(HttpMethod.Post, uploadUrlData.UploadUrl);
             using var timeoutStream = stream.ObserveReadTimeout(_timeouts.ReadWriteTimeout, false);
 
-            request.Headers.TryAddWithoutValidation("Authorization", uploadUrlData.AuthorizationToken);
+            request.Headers.Add("Authorization", uploadUrlData.AuthorizationToken);
             request.Headers.Add("X-Bz-Content-Sha1", sha1);
             request.Headers.Add("X-Bz-File-Name", _urlencodedPrefix + Utility.Uri.UrlPathEncode(remotename));
             request.Content = new StreamContent(timeoutStream, B2_RECOMMENDED_CHUNK_SIZE);
diff --git a/Duplicati/Library/Backend/Backblaze/B2AuthHelper.cs b/Duplicati/Library/Backend/Backblaze/B2AuthHelper.cs
index 0b569ede8..420b5ea01 100644
--- a/Duplicati/Library/Backend/Backblaze/B2AuthHelper.cs
+++ b/Duplicati/Library/Backend/Backblaze/B2AuthHelper.cs
@@ -81,7 +81,7 @@ public class B2AuthHelper(string userid, string password, HttpClient httpClient,
     {
         var request = await base.CreateRequestAsync(url, method, cancellationToken).ConfigureAwait(false);
         var config = await GetConfigAsync(cancellationToken).ConfigureAwait(false);
-        request.Headers.TryAddWithoutValidation("Authorization", config.AuthorizationToken);
+        request.Headers.Add("Authorization", config.AuthorizationToken);
         request.Headers.Add("User-Agent", UserAgent);
         return request;
     }
diff --git a/Duplicati/Library/Backend/Dropbox/DropboxHelper.cs b/Duplicati/Library/Backend/Dropbox/DropboxHelper.cs
index 76d69e48d..eac4c9e68 100644
--- a/Duplicati/Library/Backend/Dropbox/DropboxHelper.cs
+++ b/Duplicati/Library/Backend/Dropbox/DropboxHelper.cs
@@ -90,7 +90,7 @@ namespace Duplicati.Library.Backend
         private async Task<HttpRequestMessage> CreateChunkRequestAsync<T>(string url, T arg, CancellationToken cancelToken)
         {
             var req = await CreateRequestAsync(url, HttpMethod.Post, cancelToken).ConfigureAwait(false);
-            req.Headers.TryAddWithoutValidation(API_ARG_HEADER, System.Text.Json.JsonSerializer.Serialize(arg));
+            req.Headers.Add(API_ARG_HEADER, System.Text.Json.JsonSerializer.Serialize(arg));
             req.Options.Set(FileRequestOption, true);
             return req;
         }
@@ -165,7 +165,7 @@ namespace Duplicati.Library.Backend
         {
             var req = await CreateRequestAsync(WebApi.Dropbox.DownloadFilesUrl(), HttpMethod.Post, cancelToken).ConfigureAwait(false);
             req.Options.Set(FileRequestOption, true);
-            req.Headers.TryAddWithoutValidation(API_ARG_HEADER, System.Text.Json.JsonSerializer.Serialize(new PathArg { path = path }));
+            req.Headers.Add(API_ARG_HEADER, System.Text.Json.JsonSerializer.Serialize(new PathArg { path = path }));
 
             using (var response = await Utility.Utility.WithTimeout(m_timeouts.ShortTimeout, cancelToken, ct => GetResponseAsync(req, HttpCompletionOption.ResponseHeadersRead, ct)).ConfigureAwait(false))
             {
@@ -181,7 +181,7 @@ namespace Duplicati.Library.Backend
             {
                 using var req = await CreateRequestAsync(WebApi.Dropbox.DeleteUrl(), HttpMethod.Post, ct).ConfigureAwait(false);
                 req.Options.Set(FileRequestOption, true);
-                req.Headers.TryAddWithoutValidation(API_ARG_HEADER, System.Text.Json.JsonSerializer.Serialize(new PathArg { path = path }));
+                req.Headers.Add(API_ARG_HEADER, System.Text.Json.JsonSerializer.Serialize(new PathArg { path = path }));
                 using var response = await GetResponseAsync(req, HttpCompletionOption.ResponseContentRead, ct).ConfigureAwait(false);
             }).ConfigureAwait(false);
         }
diff --git a/Duplicati/Library/Backend/Jottacloud/Jottacloud.cs b/Duplicati/Library/Backend/Jottacloud/Jottacloud.cs
index 659dc9bfb..da33db843 100644
--- a/Duplicati/Library/Backend/Jottacloud/Jottacloud.cs
+++ b/Duplicati/Library/Backend/Jottacloud/Jottacloud.cs
@@ -594,8 +594,8 @@ public class Jottacloud : IStreamingBackend
         var fileSize = stream.Length;
         using var req = await CreateRequest(HttpMethod.Post, remotename, "umode=nomultipart", true, cancelToken).ConfigureAwait(false);
 
-        req.Headers.TryAddWithoutValidation("JMd5", md5Hash); // Not required, but it will make the server verify the content and mark the file as corrupt if there is a mismatch.
-        req.Headers.TryAddWithoutValidation("JSize", fileSize.ToString()); // Required, and used to mark file as incomplete if we upload something  be the total size of the original file!
+        req.Headers.Add("JMd5", md5Hash); // Not required, but it will make the server verify the content and mark the file as corrupt if there is a mismatch.
+        req.Headers.Add("JSize", fileSize.ToString()); // Required, and used to mark file as incomplete if we upload something  be the total size of the original file!
 
         await using var timeoutStream = stream.ObserveReadTimeout(this.m_timeouts.ReadWriteTimeout, false);
         req.Content = new StreamContent(timeoutStream);
diff --git a/Duplicati/Library/Backend/OAuthHelper/OAuthHelperHttpClient.cs b/Duplicati/Library/Backend/OAuthHelper/OAuthHelperHttpClient.cs
index 4ee7d77d1..e28456c2e 100644
--- a/Duplicati/Library/Backend/OAuthHelper/OAuthHelperHttpClient.cs
+++ b/Duplicati/Library/Backend/OAuthHelper/OAuthHelperHttpClient.cs
@@ -136,7 +136,7 @@ public class OAuthHelperHttpClient : JsonWebHelperHttpClient
 
                     return await Utility.Utility.WithTimeout(AUTHENTICATION_TIMEOUT, cancellationToken, async ct =>
                     {
-                        request.Headers.TryAddWithoutValidation("X-AuthID", _Authid);
+                        request.Headers.Add("X-AuthID", _Authid);
                         response = await _httpClient.SendAsync(request, ct).ConfigureAwait(false);
                         response.EnsureSuccessStatusCode();