Update: Well that didn't take long. It looks like 6tag is back and ready to download.
Original story: This morning, Rudy Huyn released 6tag 4.0, bringing some of the latest Instagram features to Windows Phone like new filters and finally being able to edit posts.
One issue many of you are noticing is that after you update your existing 6tag app, the program fails to load. Instead, it back out and goes back to your Start screen (or menu). Turns out it is a cache incompatibility problems and uninstalling and reinstalling the app appears to fix it.
Huyn has temporarily pulled the app from the Store as he works to patch it.
"6tag 4.0 has a problem with cached data from 6tag 3.4, an update is coming, for the moment, you can uninstall & install the app to solve it"
Your best bet if you have not yet updated is to wait a few hours when the 18.104.22.168 version hits the Store. Waiting for the fix prevents the need to re-login into Instagram and your associated social networks and alter any settings that you previously had configured.
Huyn has unpublished the app from the Store to prevent any further update woes. This removal is on purpose, so just sit tight and wait for the bug fix update to hit the Store.
Download 6tag 4.0 for Windows Phone (Temporarily unavailable)