From several conversations with Blogger Support, it now appears that this latest error code (and a few similar ones) is related to Internet Explorer, and the use of Compatibility View. Apparently, Blogger Engineering has finally updated Blogger so it is compatible with Internet Explorer V11 - and having done so, has made it incompatible with V11 using Compatibility View.
The simplest way to resolve the bx-hg9hg2 is to reset Internet Explorer.
Many Blogger blog owners may be using Internet Explorer for more than just Blogger blog maintenance - and may not appreciate the possible effect on access to other websites, caused by a browser reset. In this case, it may be better to Edit the Internet Explorer settings, and remove Blogger from Compatibility View.
To edit Compatibility View, you use the browser Tools menu, and select "Compatibility View Settings". Select any or all entries referencing "blogger.com", and hit "Remove", then "Close".
Having either reset Internet Explorer, or edited the Compatibility View list, you may next need to restart Internet Explorer.
A third possibility would be to use a different browser, until Blogger Engineering resolves all issues with IE V11 and Compatibility View. This may not happen immediately, however - and some blog owners may prefer to use IE in general. Advice to "Use a different browser" is not always appreciated, by every blog owner.