I’ve been able to reproduce (but not isolate) this.
The following after restoring the ClassicPress site to WordPress v5.3.2 and restoring all user content.
( it’s not a 100% image as I can’t justify rolling back that server.)
- Run Switch to ClassicPress
URL appears: /wp-admin/about.php?updated
(blank page, no content in source )
Loading the public-facing site in another browser, loads OK
On admin screen, nothing has changed, so reload, then prompted to upgrade database
URL appears /wp-admin/about.php
(no ‘?updated’ suffix)
‘Welcome to ClassicPress!’ displays with prompt to update to v1.1.2
update this (twice) then all OK.
I saw nothing relating to this site in the PHP or Apache logs.
Repeated with less plugins (notably no WordFence) but had the same result.
On some occasions the user facing site was a blank page briefly so I deduce it’s maintenance mode…?
A DIFFERENT WP SITE ON THE SAME SERVER FUNCTIONED AS EXPECTED
On a different site on the same server with less content and plugins, I used the same tool to switch from WordPress v5.3.2 and it worked perfectly as expected.