I’ve finally solved my failures with VisualEditor in 1.28. The wiki server (Ubuntu) was missing the php-curl package. After installation, VisualEditor works flawlessly.

I was able to diagnose this by watching the network traffic between MediaWiki and parsoid using Chrome’s developer tools (the "Network" tab). There it was... the error message "Call to undefined function curl_init()."

I’m going to file a bug report that the user-visible error message in this case can perhaps be Improved. Currently the message is just a JavaScript alert that says:

examplewiki.com replied: "http"

and no errors appear in any log files.

Thanks to everyone who helped with this issue!!

DanB