The version on /download/plesk/11 _is_ the production version of Plesk. It officially went RTM (see:
http://en.wikipedia.org/wiki/Softwar...life_cycle#RTM) about mid-day June 13 (Seattle time). Plesk 11 will proceed into GA on June 26. The goal of separating these is to allow our direct customers earlier access to final product so they can organize their own launches before we begin broadly marketing availability.
Anyhow, to the error message that you got - that sounds license related, almost as if the license was removed or was not valid to work with Plesk 11. I saw another post from you that sounds like you're using Perpetual (also known as "Purchase") licenses as opposed to lease/monthly licenses. Lease/monthly licenses work across Plesk 10.x and 11.x without changes, but Perpetual licenses have a couple requirements (does it have SUS? has the license key type been upgraded to 11.x?). So, I'd recommend checking w/ your account manager on your license and giving the upgrade another go.