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mallikharjunarao gundamallikharjunarao gunda 

custom button errors: Enter a URL that is valid and well-formed

content source: URL
URLFOR($Action.Opportunity.New,null,[clone=1,id=Opportunity.Id,retURL="/"&Opportunity.Id,00N2800000FJLEz= Opportunity.Name ],true), 

NagendraNagendra (Salesforce Developers) 
Hi mallikharjunarao gunda,

Here are few threads from the developer's community with similar issue which might help you. Mark,this as solved if this helps.

Best Regards,