Received the following error when trying to delete a list column: 

“Unable to validate data.”

Thanks to Angela Chng for the solution:

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!

Removed the search box from the application.master and the issue was resolved!