Thank you for your replies guys, but "hope & try " is not a solution, or even is not "the solution" .
I hope they could fix this issue soon...
Fortunately I have saved an address that I have used often and I have right now available.
plain and simple unusable
if you are lucky you get one order through in 100 attempts, no joke. it's being going downhill for weeks now
Just be patient and if you're lucky perhaps in the 100th attempt you'll manage to get through.

I have got also the same error message:
Error 502. Site down
"Your order may have failed. Please refresh the page and check your orders. Contact support if necessary."
"Invalid Response from Server"
But most of the time, the order was already placed / just a bit delayed.