I tried another approach:

<form method="POST">
  <button>Submit</button>
  <input name="wpEditToken">
  <input name="restore" type="checkbox">
</form>

I added this to Special:Undelete&action=submit via dev tools, pasted my CSRF token into the input, checked the “restore” checkbox, and clicked the Submit button. Same error as with the API:

Could not acquire lock for "mwstore://local-multiwrite/local-public/f/fc/ACDC_test_file_1.pdf".

I wonder if the upload cert issues are linked

The upload cert issues are T293070: upload.wikimedia.beta.wmflabs.org certificate expired (October 2021), I assume.

Yes

Well, whatever happened over there in the past 15 minutes (cough) doesn’t seem to have helped, I still get the same errors :/

@LucasWerkmeister I think it is because it has so many archived revisions (10818) that it cannot undelete. Might be same issue as T257298.

Sorry for deleting this, I didn't know it was being used.

In T293558#7435575, @dom_walden wrote:

@LucasWerkmeister I think it is because it has so many archived revisions (10818) that it cannot undelete. Might be same issue as T257298.

Yes, this is probably the cause. Unlike page deletion, undeletion is unfortunately not batched, and will generally explode whenever the page has too many revisions.

I thought the number of revisions might be a problem, but I hoped that the undelete with just one timestamp (revision) and file ID might work better – and I feel like this actually points to a second problem (the LocalLockFileError).

I’ve uploaded a new version of the file, but PDF thumbnailing seems to be broken on Beta Commons (presumably unrelated), which makes the bold “do not delete this file” notice I put in the title somewhat less effective.