Authentication stalled during set up

I am trying to set UpdraftPlus to backup to DropBox, but authentication is stalling during the set up process.

1. UpdraftPlus -> settings
2. Storage option -> Dropbox
2a. ‘After you’ve saved your settings, come back here to complete authentication.’
3. Save changes
4. ‘Follow this link to authorize…’
5. Orange page, auth.updraftplus.com: ‘To complete setup for Dropbox press the button below.’
6. ——- after hitting the ‘complete setup’ button on this page, I get “This form is not secure. Are you sure you want to submit it?”
7. —— I hit submit, and the “working” blue bar gets partway across the screen (about 10-15%) and sits there.

I have checked Dropbox, and the apps/UpdraftPlus folder has been created.

I made sure the components were up to date:
WordPress 4.9.8
UpdraftPlus 1.15.2

The problem might be:
A. My site is 11gb. UpdrsftPlus FAQ says they know of no max size issue, and have successfully been used in sites up to 12gb. I have Dropbox storage up to 1Tb, so there shouldn’t be an issue on that side. Maybe an issue in the connection though?

B. The site itself, billandsusan.net, is not https at this time, which I think caused the warning in step 6. Could lack of security cause this issue?

Thanks for any help!

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