I tried to reproduce it here: https://phabricator.wikimedia.org/project/sprint/board/1384/

Works here.

What does your browser's error console say?

Patch for review here: https://gerrit.wikimedia.org/r/#/c/261185/3

Some notes about this:
Upstream has refactored the task edit controller and created a new class called ManiphestTaskEditEngine.
See https://secure.phabricator.com/T9132
Also, see https://secure.phabricator.com/T10003 about deprecated capabilities in Maniphest.

Other major changes include a new route for the project profile edit controller and new edit engine functions for the project profile form (e.g. customization and default settings).

Error from console:

POST https://phabricator.tinyowl.com/project/sprint/board/task/edit/225/ 500 (Internal Server Error)
JX.install.members.send @ core.pkg.js:2269
JX.install.members.start @ core.pkg.js:4687
JX.install.statics._dosubmit @ core.pkg.js:4562
JX.install.statics._onbutton @ core.pkg.js:4530
JX.install.statics.pass @ core.pkg.js:1608
JX.install.statics._dispatchProxy @ core.pkg.js:1573
JX.install.statics.dispatch @ core.pkg.js:1497
JX.__rawEventQueue @ init.js:85

I pulled latest commit.
Create Task and Edit dialogs are not opening up.
Getting 500s as soon as edit icon is clicked OR create task is clicked.

POST https://phabricator.tinyowl.com/project/sprint/board/task/edit/225/ 500 (Internal Server Error)
POST https://phabricator.tinyowl.com/project/sprint/board/task/create/ 500 (Internal Server Error)

For record. I tested on this configuration:
phabricator 571960e712276083be4a0d2a67119c5aade02d25 (Sat, Dec 26)
arcanist 3dbc1418ff07de30cbd22193efad0efd5fc2d7f2 (Sat, Dec 26)
phutil a654742b2100503df00254752740455e40b72d01 (Sat, Dec 26)
sprint bce9e295f3c5e556069d3b18cf8ff06252d7a28f (Mon, Dec 28)

@Christopher: With your patch, things mostly work at https://phab-01.wmflabs.org but sprint project profiles don't seem to be working.

see https://phab-01.wmflabs.org/project/sprint/profile/20/

Exception
Bad getter call: getURI

So apparently project->getURI() has been removed upstream so this line fails. I patched it on phab-01.wmflabs.org with this change:

diff --git a/src/controller/SprintProjectController.php b/src/controller/SprintProjectController.php
index a3f628f..b60d1c1 100644
--- a/src/controller/SprintProjectController.php
+++ b/src/controller/SprintProjectController.php
@@ -168,10 +168,11 @@ abstract class SprintProjectController extends SprintController {
       $ancestors = $project->getAncestorProjects();
       $ancestors = array_reverse($ancestors);
       $ancestors[] = $project;
+      $primary_slug = $project->getPrimarySlug();
+      $primary_uri = "/tag/{$primary_slug}/";
+
       foreach ($ancestors as $ancestor) {
-        $crumbs->addTextCrumb(
-            $project->getName(),
-            $project->getURI());
+        $crumbs->addTextCrumb($project->getName(), $primary_uri);
       }
     }

@mmodell See https://secure.phabricator.com/rP2b5d4bca8a8f97c755d3be25d34ee5d77fdfc605#782208d5 where buildApplicationCrumbs() was added.

Make sure that you have pulled this upstream 10ed33052361be82cbc09884118d65ec0601bd55 commit.

It is important that Sprint is kept in sync with a specific upstream commit, otherwise things might not work.

They are actively adding the new subprojects stuff now, so breaking changes will probably happen frequently. See https://secure.phabricator.com/T10010

I couldn't understand the inner details.
Is it desired to make Sprint compatible with the current upstream stable i.e. 571960e712276083be4a0d2a67119c5aade02d25?

If not, how can I patch to make it work?

assuming that you have pulled a current or newer master already

git checkout stable
git merge 10ed33052361be82cbc09884118d65ec0601bd55

Done.

btw, I am always working off of upstream master current, because there are too many variables associated localized branch (i.e. WMF) deployment. Thus, I will reference a specific upstream commit that I have patched against which will be the absolute reference point for all associated compatibility issues.

Now sprint extension is working fine with upstream/stable
👍