Since CMS-10528, repeater fields are represented like repeater[1]/field
When a validation error occurs, the errors map is filled with an entry <"repeater[1]/field", Error>
Then the error map is given to the ActionResultGenerator which try to create an XML Element with the key name, leading to an exception in the DispatchGenerator, itself not well handled by the client, with no error dialog, and the Content Tool is closed, losing all data.