Ajax stops working in admin area with Woocommerce 4.0.1

I just finished building a very simple shop with Storefront as the theme.

After updating and launching the site, I noticed:

1. When I searched for a plugin, the spinning circle on the search page spun forever – but when I refreshed the page, search results appeared straight away.

2. Not able to add or remove plugins – error messages appeared at the top of the page, eg:

Installation failed: {“success”:true,”data”:{“install”:”plugin”,”slug”
Deletion failed: {“success”:true,”data”:{“delete”:”plugin”

3. Not able to see some data or save settings in some other plugins, eg. Wordfence (I was trying to save scan settings and view log files)

4. Disabling Woocommerce made everything work normally.

5. Disabling all other plugins did not fix the problem, so not a plugin conflict.

6. Reinstalling WordPress did not help.

7. There were lots of error logs in the server relating to admin-ajax.php, eg:

/home/websitename/public_html/wp-content/plugins/woocommerce/packages/action-scheduler/functions.php(30): Actio in /home/websitename/public_html/wp-content/plugins/woocommerce/packages/action-scheduler/classes/migration/ActionScheduler_DBStoreMigrator.php on line 44: /domains/s/c/websitename.co.uk/public_html/wp-admin/admin-ajax.php, referer: https://www.websitename.co.uk/wp-admin/admin.php?page=WordfenceScan

8. All of the things that wouldn’t work used ajax.

I found a support post from a month ago where the OP had a problem that was fixed by uninstalling 4.0 / 4.0.1 and rolling back to 3.9.x. I tried this and it worked for me. So there seems to be a problem with Woocommerce 4.0.