Added the localhost listen variant.

Duplicati/Library/RestAPI/Strings.cs

@@ -32,7 +32,7 @@ Error message: {0}", error); }
                 public static string WebserverPortDescription { get { return LC.L(@"The port the webserver listens on. Multiple values may be supplied with a comma in between."); } }
                 public static string WebserverCertificateFileDescription { get { return LC.L(@"The certificate and key file in PKCS #12 format the webserver use for SSL. Only RSA/DSA keys are supported."); } }
                 public static string WebserverCertificatePasswordDescription { get { return LC.L(@"The password for decryption of certificate PKCS #12 file."); } }
-                public static string WebserverInterfaceDescription { get { return LC.L(@"The interface the webserver listens on. The special values ""*"" and ""any"" means any interface. The special value ""loopback"" means the loopback adapter and ""ipv6loopback"" means IPv6 loopback."); } }
+                public static string WebserverInterfaceDescription { get { return LC.L(@"The interface the webserver listens on. The special values ""*"" and ""any"" means any interface. The special value ""loopback"" means the loopback adapter"); } }
                 public static string WebserverPasswordDescription { get { return LC.L(@"The password required to access the webserver. This option is saved so you do not need to set it on each run. Setting an empty value disables the password."); } }
                 public static string WebserverAllowedhostnamesDescription { get { return LC.L(@"The hostnames that are accepted, separated with semicolons. If any of the hostnames are ""*"", all hostnames are allowed and the hostname checking is disabled."); } }
                 public static string PingpongkeepaliveLong { get { return LC.L(@"When running as a server, the service daemon must verify that the process is responding. If this option is enabled, the server reads stdin and writes a reply to each line read."); } }

Duplicati/Server/WebServerLoader.cs

@@ -173,8 +173,6 @@ public static class WebServerLoader
         interfacestring = interfacestring.Trim();
         if (new[] { "*", "all", "any" }.Any(x => x.Equals(interfacestring, StringComparison.OrdinalIgnoreCase)))
             listenInterface = System.Net.IPAddress.Any;
-        else if (interfacestring == "ipv6loopback")
-            listenInterface = System.Net.IPAddress.IPv6Loopback;
         else if (interfacestring != "loopback")
             listenInterface = System.Net.IPAddress.Parse(interfacestring);
 

Duplicati/WebserverCore/DuplicatiWebserver.cs

@@ -107,6 +107,14 @@ public partial class DuplicatiWebserver
                         listenOptions.UseHttps(settings.Certificate);
                 });
             }
+            else if (settings.Interface == System.Net.IPAddress.Loopback)
+            {
+                options.ListenLocalhost(settings.Port, listenOptions =>
+                {
+                    if (settings.Certificate != null)
+                        listenOptions.UseHttps(settings.Certificate);
+                });
+            }
             else
             {
                 options.Listen(settings.Interface, settings.Port, listenOptions =>