Adding/editing shipping address redirect to registration form

[PSV]

Using Virtuemart 4.0.12 on Joomla 3.10.11 I have a problem with customer shopping addresses. Once logged in as a customer, accessing the account data works fine and the shown billing address can be edited just fine as well. Below that form are the links to allow you to add a new or edit an existing shipping addresses. The problem is those links always redirect to the login/register form, and I've no idea what causes this behavior. (Worth adding, we are using Rupostels OnePageCheckout and the OPC function to add/edit shipping addresses during the checkout process work just fine. It's only the customer's account area where this becomes a problem.)
Site is here: https://vj3.lies-ein-buch.de/ (Please note that user registration currently requires manual activation by an admin.)

Edit: Did some more digging and it seems the link is simply not found, removing a shipping address works fine. So a link like this: https://vj3.lies-ein-buch.de/ihr-konto/removeAddressST?virtuemart_user_id[0]=1234&virtuemart_userinfo_id=007&8551a65894564567577567a55a72e6fb=1 works and successfully removes the address, while the edit links looking like this: https://vj3.lies-ein-buch.de/ihr-konto/addST?virtuemart_user_id[0]=1234&virtuemart_userinfo_id=007 is more or less an 404 defaulting to the registration page. Which raises the question where does "addST" come from and what should it actually be (or why does it appear not to exist if it's correct).

Also of note in the backend the edit form shows up but is not populated with the data of the chosen address and entering data creates a new address, so editing an address does not work even in the backend, although the failure is somewhat different.
The backend links also includes "addST" though, convincing me even more that it's at least part of the problem: https://vj3.lies-ein-buch.de/administrator/index.php?option=com_virtuemart&view=user&task=addST&addrtype=ST&virtuemart_user_id[]=1234&virtuemart_userinfo_id=007

[jenkinhill]

Are you able to test with a more recent VM version? We are currently at VM 4.0.22

[Roderic]

4.0.20 you mean. I'm sure 4.0.22 is already available in some dev environment but it's not offered as a download yet on extensions.virtuemart.net

[jenkinhill]

@Roderic I try to stay ahead of member versions so I can check if reported issues seem to have been fixed.

Yes, 4.0.20 is the latest new user package with J 3.10.11    https://dev.virtuemart.net/attachments/download/1364/VirtueMart4.0.20_Joomla_3.10.11-Stable-Full_Package.zip