Advanced Blocking ranges disappeared

I entered quite a few “Advanced Blocking” ranges two days ago.

Today I logged in and discovered that all of them disappeared: the “Current list of ranges and patterns you’ve blocked” on the “Advanced Blocking” page is empty.

How could it happen?

I am the only “administrator” role user.
WP, all plugins and themes are up-to-date.
Wordfence never reported any suspicious activity.
I did get a handful spam user registrations in the last week, all by default are in “subscriber” role.

— EDIT: I just added a range to that empty list and – lo and behold! – all lost ranges from two days ago reappeared on that page.

Then re-loaded (F5) the “Advanced Blocking” page and the list is empty again. This time, the described in the above paragraph trick of adding another range does not work.

I access the admin area via SSL, if that matters.

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