About the Ubiquity part - Ubiquity expects to retrieve connectivity-check.html and expects it to have md5 4589f42e1546aa47ca181e5d949d310b. Alas, this is not currently the case, so I would expect 100% of Ubiquity users to be unable to check the “Download Updates…” checkbox. I have recovered the original connectivity-check.html content and have a request open internally to restore connectivity-check.html to match 4589f42e1546aa47ca181e5d949d310b.
Note also that for updates to be available, other parts of the network check need to be happy, so even when this is fixed, if Ubiquity deems you to be offline the checkbox will probably be disabled.
Thanks for the bug report @wbtwcx