I'm getting this error, too (screenshot below), on two different sites. Both on the same server, being served over SSL by Nginx as a reverse proxy.
Fetch-failed-JSON
The following from the browser devtools:
POST https://www.iainsgillis.com/admin/notifications.json/task:getNotifications 500 ()
o @ admin.min.js?92aed16245:4
value @ admin.min.js?92aed16245:4
(anonymous function) @ admin.min.js?92aed16245:4
t @ vendor.min.js?92aed16245:1
(anonymous function) @ admin.min.js?92aed16245:4
t @ vendor.min.js?92aed16245:1
(anonymous function) @ admin.min.js?92aed16245:1
(anonymous function) @ admin.min.js?92aed16245:1
t @ vendor.min.js?92aed16245:1
window.webpackJsonpGrav @ vendor.min.js?92aed16245:1
(anonymous function) @ admin.min.js?92aed16245:1
Unexpected token < in JSON at position 0 at SyntaxError: Unexpected token < in JSON at position 0
s @ admin.min.js?92aed16245:4
POST https://www.iainsgillis.com/admin/ajax.json/task:getnewsfeed 500 ()
o @ admin.min.js?92aed16245:4
value @ admin.min.js?92aed16245:4
value @ admin.min.js?92aed16245:4
(anonymous function) @ admin.min.js?92aed16245:4
j @ jquery-2.1.4.min.js?92aed16245:2
fireWith @ jquery-2.1.4.min.js?92aed16245:2
ready @ jquery-2.1.4.min.js?92aed16245:2
I @ jquery-2.1.4.min.js?92aed16245:2
Unexpected token < in JSON at position 0 at SyntaxError: Unexpected token < in JSON at position 0
s @ admin.min.js?92aed16245:4
These errors occur on any save/refresh/navigation within the admin panel.
There's an additional instance of the same error when clearing the cache from within the admin panel:
GET https://www.iainsgillis.com/admin/cache.json/task:clearCache/admin-nonce:89a9b7f7b63b53d3ba786748d5c5fe25 500 ()
o @ admin.min.js?71de152ee2:4
value @ admin.min.js?71de152ee2:4
(anonymous function) @ admin.min.js?71de152ee2:4
dispatch @ jquery-2.1.4.min.js?71de152ee2:3
r.handle @ jquery-2.1.4.min.js?71de152ee2:3
I've checked the source of the render pages: nothing is being output before the initial <!DOCTYPE html>. Also: PHP memory limit is set to 128M.
Somehow related, perhaps: trying to update a plugin from the admin panel also hangs, with the server returning a 404 error for /admin/plugins.json/task:getPackagesDependencies. Similarly, the server returns a 500 error for /admin/backup.json/task:backup/admin-nonce:89a9b7f7b63b53d3ba786748d5 c5fe25.
Could all of these be related to the proxy setup? No problems whatsoever when doing any of the updates, backups, or cache-clearing from the CLI.
Hope this is helpful.