This happened to me as well. I was in ropsten.
I noticed when I started the geth client directly and then tried to access the Mist wallet, was getting this error. But when I just started the Mist wallet directly without starting a geth client then was able to access the wallet UI.
I then tried initializing the geth with a new genesis config file. After doing that started the geth with a new networkid and on trying to access the Mist i was able to open it. So didn't face the error anymore
When I started with a different networkid starting with number different from 1 could again access my wallet.