Security warning (encryption) while checking out - OK or...?

I just tried to apply the monthly PA coupon to my cart before checking out, and got a "Security Warning" popup when I clicked the coupon Apply button.
It's the old familar one about sending data from an encrypted page over an unencrypted connection. I used to see this every now and then on random other sites, but it's been quite a while since the last time it showed up.
Is this an actual problem, and could it be a store page glitch in the coding of the Apply button? All indications are that the cart page has security and encryption properly set up. I didn't see the error the last time I tried applying a coupon a couple of days ago.
Using Win7 64-bit and Firefox, both current and up-to-date.
Comments
It's perfectly safe: http://www.daz3d.com/forums/viewreply/712040/
Are you sure that's the same problem? I have logged in securely, the cart page itself is secure, every single My Account page is secure, it's just that the code associated with the Apply Coupon button does not send the code on an encrypted connection. See DAZ_Jon's reply at the end of that thread, it's the opposite of what I'm seeing here.
Edit: Just tried again an hour later, and this time it worked with no glitches. Something was definitely going wrong there, though.