looked at unity's AsyncOperation.progress which is apparently terribly broken so the download ProgressBar now ignores negative progress changes.

Assets/RothenburgAR/Scripts/Updater/UpdaterBehaviour.cs

@@ -208,8 +208,6 @@ namespace RothenburgAR.Updater
 
         private void UpdateMedia(Exhibit exhibit)
         {
-            //TODO check existing file for creation/alteration date to see if download is necessary
-
             var mediaIDs = exhibit.Pois.Select(p => p.MediaId).Except(new List<string> { null }).ToList();
 
             foreach (var mediaId in mediaIDs)
@@ -217,7 +215,7 @@ namespace RothenburgAR.Updater
                 var path = PathHelper.CombinePaths(PathHelper.MediaPath, mediaId);
                 var filepath = Path.Combine(path, "media.zip");
 
-                //TODO cancel download if unnecessary
+                //TODO cancel download if unnecessary, how to check if unnecessary?
                 if (File.Exists(filepath)) continue;
 
                 // create file and dir if nonexistent
@@ -286,7 +284,12 @@ namespace RothenburgAR.Updater
 
                 downloadProgress /= httpHandlers.Count;
 
-                ProgressBar.value = downloadProgress;
+                // operation.progress is bugged and goes all over the place:
+                //      jumps from 0 to 1 and back despite not being done 
+                //      steadily increases from 0,5 to 0,99 and resets to 0,5 multiple times (5-15x) during a single download
+
+                // update progress bar and ignore all negative changes
+                ProgressBar.value = ProgressBar.value < downloadProgress ? downloadProgress : ProgressBar.value;
             }
 
             // Continue to Main Scene after all downloads are done and this.afterDownloadWaitTime seconds have passed without any new downloads triggering.