With the DDO Store changes in Update 50.4, the DDO Store is now inaccessible on systems running Windows Vista.
Prior to the update, the DDO Store was only accessible on Windows Vista systems via external browser.
There's probably no workaround, due to DDO announcing earlier that it would no longer support Windows Vista. Posting in case anyone has ideas.
(I need to get my hands on a copy of Windows 10 to reformat, which would be my best option. Health and finances have delayed that.)
(Edit) Here is the error message that shows up in the DDO Store when it opens in the browser page:
Something is going wrong
It's not you, it's us, but we can help. Please contact our customer support.
Error code: 1000-0002