Unable to add/edit a Server Object Extension

Question

I have a C# project that creates a SOE for ArcGIS Server 10.2.2. I build it in Visual Studio Express 2010 and am rewarded with MyPitaExtension.soe. The contents of that file are what I expect (Config.xml alongside an "Install" folder containing MyPitaExtension.dll & MyPitaExtension.pdb).

I go to the ArcGIS Server Manager > Sites > Extensions, and try to add/edit the SOE (in some instances it is already installed; in others it is not).
In the dialog, I choose MyPitaExtension.soe and click Add or Save. The dialog remains open and displays text below the Save/Cancel buttons that fits the format:

fjsontoken[Big ugly thing that runs off the available space]

Clicking Add or Save again adds an identical string to the bottom of the dialog.
I've tried googling various combinations of 'fjsontoken', 'esri', 'soe', etc... but all I've found was this very recent post about Portal.
I am a member of the Administrator role.

Has anyone had any luck diagnosing & fixing this?

UPDATE 1: A coworker (also in an Admin) gets the same results as I do.

UPDATE 2: I get the same results when adding NetSimpleRESTSOE.soe pre-packaged with the ArcObjects SDK. I'm now confident that this is a problem with our server configuration rather than a problem with the SOE itself.

Answer 1

The geonet post linked above contains an answer that I just plain missed (I've been trying to cut back on my caffiene). That answer referenced a known bug. The way to avoid this is to use MSIE when uploading SOE files. I used Chrome and my coworker used firefox.

UPDATE: At least in v10.4.1, MSIE is also required for creating new data stores. Also there appears to be a patch for this now (which is already behind Chrome's latest release).