hofix: https://github.com/nextcloud/server/pull/110

src/com/owncloud/android/datamodel/ThumbnailsCacheManager.java

@@ -285,6 +285,7 @@ public class ThumbnailsCacheManager {
                                         px + "/" + px + Uri.encode(file.getRemotePath(), "/");
                                 Log_OC.d("Thumbnail", "URI: " + uri);
                                 get = new GetMethod(uri);
+                                get.setRequestHeader("Cookie", "nc_sameSiteCookielax=true;nc_sameSiteCookiestrict=true");
                                 int status = mClient.executeMethod(get);
                                 if (status == HttpStatus.SC_OK) {
                                     InputStream inputStream = get.getResponseBodyAsStream();