First of all the link format in your example is not quite right, though it still works. It should be
xmpp:user@server?message, not just xmpp:user@server
I was able to reproduce your initial issue (a link to a Store) when i have removed xmpp handler from the registry. So it looks like Spark didn't register uri handler for some reason. And you ahve fixed this by importing the reg file. It did register on my Win 8.1 system and i didn't have to import anything. Maybe you 2.7.0 build had a bug. I'm using the latest version from here https://igniterealtime.org/downloads/nightly_spark.jsp
I think that you are getting a blank tab because of that "target="_blank"" in your code example. This part is not needed and you can ask your webpage developers to remove it.
But this won't help much, because of the Spark bug i have mentioned. It still opens wrong chat window (with a "null" person).
I have tested this with Pidgin and it works fine indeed.
