User agreements page can't be interacted anymore once the consent modal gets closed

To reproduce:

1. Log in as admin
2. Go to Site administration / Users / Privacy and policies / Policy settings
3. Set the "Site policy handler to tool_policy.
4. Save the changes.
5. Go to Site administration / Users / Privacy and policies / Manage policies
6. Create a site policy and make it active.
7. Back to the "Policies and agreements" page, agreements link for the site policy.
8. Tick one or more users and click on the "Consent" button at the bottom of the table.
9. When the modal shows up, click "Cancel" or click outside the modal to close it.
10. Click again on any of the checkboxes.
11. Expected: You can continue selecting/deselecting users.
12. Actual: The page freezes up and you can't do anything anymore unless you refresh the page.

I thought at first that it could be a regression from MDL-60207. But since I can reproduce this as well even in 35, then the problem is most likely in the "User agreements" page.