"Application error when access /_layouts/mngfield.aspx, Error=Unable to validate data. at System.Web.Configuration.MachineKeySection.GetDecodedData(Byte buf, Byte modifier ..."
I got this one when I tried to make changes to a field on a list (mngfield.aspx).
The reason in my case whas that the master page included a
Others have discovered the same error: Read here