Qt 5.5.x doesn't know how to handle redirects.

src/Vehicle/MavlinkLogManager.cc

@@ -460,7 +460,9 @@ MavlinkLogManager::_sendLog(const QString& logFile)
     multiPart->append(logPart);
     file->setParent(multiPart);
     QNetworkRequest request(_uploadURL);
+#if QT_VERSION > 0x050600
     request.setAttribute(QNetworkRequest::FollowRedirectsAttribute, true);
+#endif
     QNetworkReply* reply = _nam->post(request, multiPart);
     connect(reply, &QNetworkReply::finished,  this, &MavlinkLogManager::_uploadFinished);
     connect(this, &MavlinkLogManager::abortUpload, reply, &QNetworkReply::abort);

src/ui/preferences/MavlinkSettings.qml

@@ -29,18 +29,24 @@ Rectangle {
     property real _valueWidth:          ScreenTools.defaultFontPixelWidth * 24
     property int  _selectedCount:       0
     property real _columnSpacing:       ScreenTools.defaultFontPixelHeight * 0.25
-
+    property bool _uploadedSelected:    false
 
     QGCPalette { id: qgcPal }
 
     Connections {
         target: QGroundControl.mavlinkLogManager
         onSelectedCountChanged: {
+            _uploadedSelected = false
             var selected = 0
             for(var i = 0; i < QGroundControl.mavlinkLogManager.logFiles.count; i++) {
                 var logFile = QGroundControl.mavlinkLogManager.logFiles.get(i)
-                if(logFile.selected)
+                if(logFile.selected) {
                     selected++
+                    //-- If an uploaded file is selected, disable "Upload" button
+                    if(logFile.uploaded) {
+                        _uploadedSelected = true
+                    }
+                }
             }
             _selectedCount = selected
         }
@@ -425,7 +431,7 @@ Rectangle {
                         }
                         QGCButton {
                             text:       "Upload Selected"
-                            enabled:    _selectedCount > 0 && !QGroundControl.mavlinkLogManager.uploading && !QGroundControl.mavlinkLogManager.logRunning
+                            enabled:    _selectedCount > 0 && !QGroundControl.mavlinkLogManager.uploading && !QGroundControl.mavlinkLogManager.logRunning && !_uploadedSelected
                             visible:    !QGroundControl.mavlinkLogManager.uploading
                             onClicked:  {
                                 QGroundControl.mavlinkLogManager.emailAddress = emailField.text