skater coder

Did you customize your application master pages and get this error while trying to delete list columns?

Unable to validate data. at System.Web.Configuration.MachineKeySection.GetDecodedData(Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Int32& dataLength)  
 
at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString)  

Ok that probably means you have a search box in the template which is messing things up, just remove it or hide it, and that will do the trick!