In a demo instance previous to production release, I created an automated action to avoid VAT duplication and got a message where Warning is not recognized in the execution phase!!! There is no message of a syntaxis error. Next the code:

if record.vat:
   contact = env ['res.partner'].search([['vat','=',record.vat]])
   if contact:
     for c in contact:
         if c.id != record.id:
            raise Warning ('VAT is duplicated, change it pls')

Any idea to solve this problem? This was working properly before the last version update