Upgrade 2.50 to 2.80 Admin Panel problem

I migrate from 2.50 to 2.80.
When Login in admin panel and try to change something NopCommerce return to login page all time.

https://www.nopcommerce.com/boards/t/22869/registration-error.aspx

Thanks Miriann,

but in what file can i put the machinekey. I have web.config in root directory and Administrarion/Views, and some more directories.

You can tell my in wich one.

Thansk Miriann,

I settings in IIS in MachineKey:

Encryption Method = SHA1
Decryption Method = AES
In validation key and decryption key section
Automatic generate at runtime = checked
Generate a unique key for each application = checked

And works.

Thanks