Can't Add New Theme

Are you trying to go through the repository (library of themes) or uploading a new theme?

I’m not sure what you mean. On my web site WP dashboard there is under “Appearance” the “Theme” category. I want to add a new theme. When I click on the “+” button, I get an error. This did not happen prior to upgrading to WP 4.4.

There have been several issues following the upgrade to 4.4 I haven’t had any personally but I’ve had several clients come to me for help on issues similar to yours.

If you want to add a new theme, you should do it old school via FTP upload. This is very simple. I use Filezilla for mine but you can use whichever you like. Check with your host on your IP address and password, go under wp-content > themes to upload your theme. If you’re needing a little bit more help you can check out any youtube vid on ftp wordpress upload

I’m not so much interested in the old school approach. My issue is that something that worked, now does not after “upgrade”.
Does anyone know if this is on WP’s radar?

This issue happened to me within the last 48 hours, and it has evolved. I’m also now unable to perform live previews and then activate new themes, nor, as you describe, am I able to add new themes. All the links on the Add Theme page are dead and I can’t see any themes to browse through or add. Reinstallation provides a short fix, but after each new install, the problems described escalate progressively faster than each time before.

Looking at my install from within their internal network, my host’s “support team” sees no problem, although their customer service does. The issue manifests on Safari for OS X and iOS, Firefox for OS X and Windows, Chrome for Windows, and Puffin Pro for iOS.