Debugging iOS app

We have an iOS app in production that suddenly started crashing when users perform a task. The app stopped working unexpectedly, so we think perhaps a call to an external library or service is the cause. The developer that coded the app is no longer with the project, and we do not have anyone in house that can look into it. We would need you to download the app code from our Git repository, and with our guidance and explanations debug it to see why it is now crashing, and help produce the necessary fix and re-upload to the app store.

If the collaboration is a good experience, we most probably would continue working together to make additional fixes on the app or develop new features.