Sitting next to a person who tries to update from 1.26 to 1.31.1, in web browser on page /mw-config/?update, step Upgrade existing installation.

Last two lines of output are

..site_stats is populated...done.
Purging caches... done.

That had been shown for at least 10min and it was unclear what to do. No errors printed above, in the web browser. Just stuck.

In the sidebar on the right, "Upgrade existing installation" is in bold. All following items are greyed out, also the "Complete!" item.
No buttons nowhere, only a "Start installation again" link at the bottom of the side bar. Hence user does not know what to do.

After pointing to https://www.mediawiki.org/wiki/Manual:How_to_debug#Logging we realized that there were some internal errors with Wikibase, but I'd still expect the UI to be less confusing/frustrating.