Dealing with the evolution of operating systems is challenging for developers of mobile apps, who have to deal with frequent upgrades that often include backward incompatible changes of the underlying API framework. As a consequence of framework upgrades, apps may show misbehaviours and unexpected crashes once executed within an evolved environment.Identifying the portion of the app that must be modified to correctly execute on a newly released operating system can be challenging. Although incompatibilities are visibile at the level of the interactions between the app and its execution environment, the actual methods to be changed are often located in classes that do not directly interact with any external element.To facilitate debugging activities for problems introduced by backward incompatible upgrades of the operating system, this paper presents FILO, a technique that can recommend the method that must be changed to implement the fix from the analysis of a single failing execution. FILO can also select key symptomatic anomalous events that can help the developer understanding the reason of the failure and facilitate the implementation of the fix.Our evaluation with multiple known compatibility problems introduced by Android upgrades shows that FILO can effectively and efficiently identify the faulty methods in the apps.