Issue: Upgrader stuck at 1%

If there are no errors displayed in the Javascript console, try visiting step=1 in the URL. It might give you some clue as to what is happening.

The character encoding of a framed document was not declared. The document may appear different if viewed without the document framing it. index.html
XenForo.activate(%o) [object XrayWrapper [object HTMLDocument]] xenforo.js:221
"XenForo.init() %dms. jQuery 23/1.11.0" "1.2.8-dev" xenforo.js:209

That message is true, but I can't get the script to crash as a result. I'm not even seeing the message myself.
You may have to use manual mode by visiting step=1. If manual mode works, then please let me know what browser you were trying that was getting this console message.

You mean &step=1
This works!

Leaving this open until the character encoding issue is resolved. Further, I think there should be a button to enter manual mode if the automatic script gets stuck.

I think this was originally caused by the same issue here: https://www.vaultwiki.org/issues/3769/

The Javascript file that set security info for asynchronous requests was empty on your installation, which led to a security error. Since this occurred as the very first step in the upgrade, it would be completely invisible in the progress window unless you snooped around the XHR response.

Would mark this as duplicate except for my posts above: there should be better resilience to errors in the installer/upgrader, so that one can at least switch manual<-->automatic at will to see what's going on behind the scenes.

Added declaration of the character encoding for the progress window.

Marking as fixed in the next release. In the next release, install/upgrade will have a linked notice at the bottom: "If it seems stuck, try switching to Manual Mode" and respectively "If it's going well, try switching to Auto Mode."

Also added a Manual Mode for the importer. Likewise, the appropriate mode switches have been added.