Unable to load assets and asset organize page is blank

Recently the ability to upload assets has stopped functioning on one of my sites. When I click "Select or upload an image", the pop-up "Select an Asset" panel shows with all assets that have previously been uploaded, but the buttons for "Add asset" and "Use selected" aren't functional -- clicking them does nothing.

In addition when I go the Organize > Assets page, no assets appear.

When I'm in the entry page I get two console errors in the browser:

[Error] Failed to load resource: the server responded with a status of 500 (Internal Server Error) (asset-filter, line 0)
[Error] Failed to load resource: the server responded with a status of 500 (Internal Server Error) (asset-filter, line 0)

My diagnostic information is below:

Viewing diagnostic information

Perch Runway: 3.1.2, PHP: 7.0.30, MySQL: mysqlnd 5.0.12-dev - 20150407 - $Id: b5c5906d452ec590732a93b051f3827e02749b83 $, with PDO
Server OS: Linux, cgi-fcgi
Installed apps: content (3.1.2), assets (3.1.2), categories (3.1.2), perch_blog (5.6.1), perch_varnish (1.0), pipit_sharing (1.2.0)
App runtimes: <?php $apps_list = array( 'perch_blog', 'pipit_sharing', 'categories', ); ?>
PERCH_LOGINPATH: /perch
PERCH_PATH: /home/tswan/oviahealth.com/perch
PERCH_CORE: /home/tswan/oviahealth.com/perch/core
PERCH_RESFILEPATH: /home/tswan/oviahealth.com/perch/resources
Image manipulation: GD Imagick
PHP limits: Max upload 64M, Max POST 65M, Memory: 90M, Total max file upload: 64M
F1: 3b606135b33e6a102526838f4152a807
Resource folder writeable: Yes
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php
REQUEST_URI: /perch/core/settings/diagnostics/
DOCUMENT_ROOT: /home/tswan/oviahealth.com
HTTP_HOST: www.oviahealth.dreamhosters.com

When I turn on debug for an entry page, I get:

Debug Message - Perch Runway 3.1.1
[1]
SELECT u.*, r.* FROM perch3_users u, perch3_user_roles r WHERE u.roleID=r.roleID AND u.userEnabled=1 AND u.userID=1 AND u.userHash='7e1dd31ff47501ba6c3e4d8d0311013c' LIMIT 1
UPDATE perch3_users SET userHash='e8f7bbed2b9a9e7ed59c6ba7ca70fd0a' WHERE userID='1'
[34]
SELECT settingID, settingValue, userID FROM perch3_settings WHERE userID=1 OR userID=0 ORDER BY userID ASC
Adding: Varnish
Varnish enavled!
Adding: Pipit Sharing
[12]
SELECT itemValue FROM perch3_menu_items WHERE itemType='app'
[0]
SELECT COUNT(*) FROM perch3_blog_comments WHERE commentStatus='PENDING'
[1]
SELECT * FROM perch3_blog_authors WHERE authorEmail='timswan@gmail.com' LIMIT 1
[1]
SELECT * FROM perch3_blog_posts WHERE postID=296
[1]
SELECT * FROM perch3_blog_sections ORDER BY sectionTitle ASC
[3]
SELECT * FROM perch3_blogs ORDER BY blogTitle ASC
[1]
SELECT * FROM perch3_blogs WHERE blogID=1 LIMIT 1
[1]
SELECT * FROM perch3_blog_sections WHERE blogID='1' ORDER BY sectionTitle ASC
Using template: /templates/blog/post.html
Using sub-template: /templates/content/blocks/blog/text_block.html
Using sub-template: /templates/content/blocks/blog/text-over-3-links.html
Using sub-template: /templates/content/blocks/blog/pullquote.html
Using sub-template: /templates/content/blocks/blog/full-width-image.html
Edit mode: edit
[1]
SELECT mi.*, p.privKey FROM perch3_menu_items mi LEFT JOIN perch3_user_privileges p ON mi.privID=p.privID WHERE mi.itemActive=1 AND mi.parentID=0 ORDER BY itemOrder ASC LIMIT 0, 1
[5]
SELECT mi.*, p.privKey FROM perch3_menu_items mi LEFT JOIN perch3_user_privileges p ON mi.privID=p.privID WHERE mi.itemActive=1 AND mi.parentID=1 ORDER BY mi.itemOrder ASC
[1]
SELECT itemTitle FROM perch3_menu_items WHERE itemType='app' AND itemValue='perch_blog' LIMIT 1
[1]
SELECT mi.*, p.privKey FROM perch3_menu_items mi LEFT JOIN perch3_user_privileges p ON mi.privID=p.privID WHERE mi.itemActive=1 AND mi.parentID=0 ORDER BY itemOrder ASC LIMIT 1, 10
[2]
SELECT mi.*, p.privKey FROM perch3_menu_items mi LEFT JOIN perch3_user_privileges p ON mi.privID=p.privID WHERE mi.itemActive=1 AND mi.parentID=2 ORDER BY mi.itemOrder ASC
File: /addons/apps/perch_blog/modes/edit.post.php
[8]
SELECT * FROM perch3_categories c, perch3_category_sets s WHERE c.setID=s.setID AND s.setSlug='blog' ORDER BY catTreePosition ASC
[1]
SELECT * FROM perch3_categories WHERE catID=10 LIMIT 1
Queries: 19
Memory: 1.2143

Any idea what could be causing this?

Thanks, Tim