If you have uploaded OS and facing this issue, this can be solved. I did it myself.
You dont have to downgrade your OS OR reset your phone.
Go to Settings>emails+accounts
Hope you will see "Attention required" error on your mail account. If YES, perform following steps.
Hope you could have registred your marketplace/live id with any e-mail account initially, you have to remeber password for that account.
you can see your e-mail id from the above option(Settings>emails+accounts).
Go to your LIve ID account & try to login with the credentials. Once you login, click on Verify your email address.
get it varified from your e-mail account which you have used. in my case I used gmail id.
Once this is done change the password of your account in windows live id site.
Settings>emails+accounts>Windows Live>provide new password & try to login.
be sure you are connected to internet, it will update & it will work fine, then you will be able to all those thisngs from marketplace.
If you have used the dummy mail ID(as it was not madatory with previous version of Mango), create the same e-mail account in windows live id.