Can't invoke assets modal for image within Perch (3) blocks

I'm experiencing exactly the same problem described by Paul and Cormac here: https://forum.grabaperch.com/forum/04-12-2017-perch-blocks-select-or-upload-an-image-wont-engage-the-asset-library

i.e. When I click the "Select or upload an image" or "Select or upload a file" links when they're inside a block, the assets overlay "flashes" (it shows up for a fraction of second and closes) and it locks the page scroll (by not removing "dialog-open" class from the body tag). It happens in both Chrome (57) and Firefox (52) for Mac.

Just logging this as a separate thread with my summary diagnostics below...

Summary information

Perch Runway: 3.0.1, PHP: 5.6.28, MySQL: mysqlnd 5.0.11-dev - 20120503 - $Id: 76b08b24596e12d4553bd41fc93cccd5bac2fe7a $, with PDO
Server OS: Darwin, apache2handler
Installed apps: content (3.0.1), assets (3.0.1), categories (3.0.1), perch_blog (5.5.1), perch_forms (1.9)
App runtimes: <?php $apps_list = [ 'perch_blog', 'perch_forms', ];
PERCH_LOGINPATH: /perch
PERCH_PATH: /Users/adam/Sites/hiddenireland.com/site/perch
PERCH_CORE: /Users/adam/Sites/hiddenireland.com/site/perch/core
PERCH_RESFILEPATH: /Users/adam/Sites/hiddenireland.com/site/perch/resources
Image manipulation: GD
PHP limits: Max upload 32M, Max POST 32M, Memory: 128M, Total max file upload: 32M
F1: 0c66c2e1f82f9e0b7617b2cb8270f2c7
Resource folder writeable: Yes
HTTP_HOST: hiddenireland.dev
DOCUMENT_ROOT: /Users/adam/Sites/hiddenireland.com/site
REQUEST_URI: /perch/core/settings/diagnostics/
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php