Wanted to say this here...
I was actually having this issue, and then I found the same issue on the main page of the support site with 2 solutions. I did the second solution (which didn't do anything for me), and then I went back and performed the first step, and now my launcher is working perfectly (and quickly).
Don't know if you need to do both the fixes in conjunction, or what - but it works, so give it a try.
Link: http://support.enmasse.com/tera/unable-to-download-retrieve-manifest
I was actually having this issue, and then I found the same issue on the main page of the support site with 2 solutions. I did the second solution (which didn't do anything for me), and then I went back and performed the first step, and now my launcher is working perfectly (and quickly).
Don't know if you need to do both the fixes in conjunction, or what - but it works, so give it a try.
Link: http://support.enmasse.com/tera/unable-to-download-retrieve-manifest