Problems with address entry and change - popup

-- HikaShop version -- : 3.5.1
-- Joomla version -- : 3.8.10
-- PHP version -- : 7.1.1
-- Browser(s) name and version -- : doesn't matter
-- Error-message(debug-mod must be tuned on) -- : no error message

On the checkout page, there is a list of customer addresses. New addresses can be added, existing ones can be changed. When clicking on a pencil to change an address, a new page opens with an entry form. This page is unstyled, it should probably appear as a popup.

When clicking "New address", there is actually a popup with a form. But after finishing and clicking OK, the popup does not close. Instead, it continues to another page within the popup, and then the whole site can be browsed inside the popup.

If the popup is closed after adding an address, it just closes, the cart page is not updated, so the new address does not show up until the page is reloaded - confusing.

Screenshot of form popup: cl.ly/tQ1K

EDIT: To make sure it is not just our template, I switched to Protostar - same results.
EDIT2: Failing popup on address change was my mistake - fixed. Problems with bad popup content and non-updating cart page remain.

Thanks for your reply.

How can I send you a private message for a logn? Otherwise you could just create an account as a client in the cart.

The process is then:
1. add product to cart
2. go to cart
3. register
4. log in
5. add or change address (URL: www.sternphila.ch/kasse/kasse/task-step/step-1 )

Regards, Ralf

Hi,

Thank you, that helped. I had to redo some of the styling, but it's ok now.

Just one issue: Error messages now appear twice: cl.ly/0v40340C1L1p
Would you have a fix for that?
Thanks!

Thanks, works, great!