After upgrading from 188.8.131.52 to 2.2.3 input validation fails (i.e. invalid input values), and returns Action.NONE instead of Action.INPUT. Log (TRACE-level) doesn't show anything interesting. Completely removing result-definition does not result in "result not defined-error". Downgrading to 184.108.40.206 fixes the problem again.
Action/Controller extends ActionSupport and implements ModelDriven<Object>. Results are defined using annotations. After debugging the Struts2-code it seems as if result is null at line 295 in RestActionInvocation.java.