diff --git a/src/libsync/discoveryphase.cpp b/src/libsync/discoveryphase.cpp
index f88b1a1db..6e95e0dbe 100644
--- a/src/libsync/discoveryphase.cpp
+++ b/src/libsync/discoveryphase.cpp
@@ -300,10 +300,8 @@ void DiscoverySingleDirectoryJob::abort()
     }
 }
 
-static std::unique_ptr<csync_file_stat_t> propertyMapToFileStat(const QMap<QString, QString> &map)
+static void propertyMapToFileStat(const QMap<QString, QString> &map, csync_file_stat_t *file_stat)
 {
-    std::unique_ptr<csync_file_stat_t> file_stat(new csync_file_stat_t);
-
     for (auto it = map.constBegin(); it != map.constEnd(); ++it) {
         QString property = it.key();
         QString value = it.value();
@@ -316,10 +314,13 @@ static std::unique_ptr<csync_file_stat_t> propertyMapToFileStat(const QMap<QStri
         } else if (property == "getlastmodified") {
             file_stat->modtime = oc_httpdate_parse(value.toUtf8());
         } else if (property == "getcontentlength") {
+            // See #4573, sometimes negative size values are returned
             bool ok = false;
             qlonglong ll = value.toLongLong(&ok);
             if (ok && ll >= 0) {
                 file_stat->size = ll;
+            } else {
+                file_stat->size = 0;
             }
         } else if (property == "getetag") {
             file_stat->etag = Utility::normalizeEtag(value.toUtf8());
@@ -346,7 +347,6 @@ static std::unique_ptr<csync_file_stat_t> propertyMapToFileStat(const QMap<QStri
             }
         }
     }
-    return file_stat;
 }
 
 void DiscoverySingleDirectoryJob::directoryListingIteratedSlot(QString file, const QMap<QString, QString> &map)
@@ -374,12 +374,26 @@ void DiscoverySingleDirectoryJob::directoryListingIteratedSlot(QString file, con
             file = file.remove(0, 1);
         }
 
-
-        std::unique_ptr<csync_file_stat_t> file_stat(propertyMapToFileStat(map));
+        std::unique_ptr<csync_file_stat_t> file_stat(new csync_file_stat_t);
         file_stat->path = file.toUtf8();
-        if (file_stat->etag.isEmpty()) {
-            qCCritical(lcDiscovery) << "etag of" << file_stat->path << "is" << file_stat->etag << "This must not happen.";
+        file_stat->size = -1;
+        file_stat->modtime = -1;
+        propertyMapToFileStat(map, file_stat.get());
+        if (file_stat->type == ItemTypeDirectory)
+            file_stat->size = 0;
+        if (file_stat->type == ItemTypeSkip
+            || file_stat->size == -1
+            || file_stat->modtime == -1
+            || file_stat->remotePerm.isNull()
+            || file_stat->etag.isEmpty()
+            || file_stat->file_id.isEmpty()) {
+            _error = tr("The server file discovery reply is missing data.");
+            qCWarning(lcDiscovery)
+                << "Missing properties:" << file << file_stat->type << file_stat->size
+                << file_stat->modtime << file_stat->remotePerm.toString()
+                << file_stat->etag << file_stat->file_id;
         }
+
         if (_isExternalStorage && file_stat->remotePerm.hasPermission(RemotePermissions::IsMounted)) {
             /* All the entries in a external storage have 'M' in their permission. However, for all
                purposes in the desktop client, we only need to know about the mount points.
@@ -414,6 +428,10 @@ void DiscoverySingleDirectoryJob::lsJobFinishedWithoutErrorSlot()
         emit finishedWithError(ERRNO_WRONG_CONTENT, QLatin1String("Server error: PROPFIND reply is not XML formatted!"));
         deleteLater();
         return;
+    } else if (!_error.isEmpty()) {
+        emit finishedWithError(ERRNO_ERROR_STRING, _error);
+        deleteLater();
+        return;
     }
     emit etag(_firstEtag);
     emit etagConcatenation(_etagConcatenation);
diff --git a/src/libsync/discoveryphase.h b/src/libsync/discoveryphase.h
index b5e743d33..238f1e822 100644
--- a/src/libsync/discoveryphase.h
+++ b/src/libsync/discoveryphase.h
@@ -91,6 +91,8 @@ private:
     bool _isRootPath;
     // If this directory is an external storage (The first item has 'M' in its permission)
     bool _isExternalStorage;
+    // If set, the discovery will finish with an error
+    QString _error;
     QPointer<LsColJob> _lsColJob;
 
 public: