This is a really strange issue. I'm testing a large production Woocommerce site with a 2 gig database. The production site needs updates to Woocommerce and plugins, so I want to test on staging. I can add users on the production site, and Woocommerce subscriptions work.
I copy the database to a testing subdomain using mysqldump and change all URLs in the database using wp-cli. Everything on the staging site works - sandbox checkout of current users, etc - except adding users via New User in admin. This also means Woocommerce subscriptions don't work, as that process adds a user as part of the checkout. (Yes, Woocommerce subscriptions are correctly enabled as a sandbox for testing on staging.)
When I try adding a new user in admin, the page loads for a few seconds and I see the green flagged WP alert that the user has been added. But when I search for the user in admin and in PHPMyAdmin, the user hasn't been added. The site admin gets an email that a user has been added, but the username and email are blank.
There are no errors in debug, no errors when using a query monitor, no errors in the general server php error log or in the MySQL or slow query log.
On staging, I've tried manually replacing all WP core files/folders, deactivating all plugins and using the 2019 theme.
I've raised max_input_vars and post_max_size in PHP7.2.
Server is Maria 10.2. I've repaired all database tables.
I can add users using a query directly in PHPMyAdmin.
Trying to add a user with wp-cli
wp user create username email@example.com --role=subscriber
results in an "Error: Unknown error creating new user"
When I create a new database on staging and install WP as a "new" site with the same WP files/folders and plugins, I can add new users in admin. But as soon as I go back to the working database copied from production, I can't add users.