This is probably caused by that fact that Fabric SDK's version is incompatible with Crashlytics SDK's version. I happened to stumble on this pitfall when I ran 'pod update Fabric'.
This updated the Fabric SDK but not the Crashlytics. And the problem occurred.
However, Crashlytics SDK's podspec file specifies a dependency on Fabric and also the compatible versions of this dependency. This is what I assumed from the Podfile.lock file:
- Crashlytics (3.8.6):
- Fabric (~> 1.6.3)
So, if the version range specification '(~> 1.6.3)' is correct, even though I ran 'pod update Fabric', this problem shouldn't occur. However, it did. So, my conclusion is the version range specification contains incompatible versions.
My solution is to run 'pod update Crashlytics' only and updated the Crashlytics and also its dependencies.
If this doesn't solve your problem, try removing Fabric and Crashlytics and adding the latest version back.