To reproduce:

1. Download and open attached "438308_cacheExpiration.zip" project.
2. Build and run the project on any platform (Android/iOS/Standalone).
3. When project loads, press "Load" button to load assetbundle.
4. After it is loaded, in "Clear Delay" input field enter a time after which the cache will be cleared. For example 2.
5. Tick the "Clear with delay" checkbox.
6. Press "Clear Cache" button.

Expected: Application does not crash when cleaning cache.
Actual result: Application crashes when trying to clean cache with delay.

Reproduced with: 5.6.4p2, 5.6.4p4.

Notes: In 2017 CleanCache method was replaced with ClearCache and therefore it does not crash anymore. Issue did reproduce on iOS/Android/Desktops and did not reproduce with WebGL. Issue only happens when CleanCache method is overloaded with "expiration" parameter. Cleaning cache with non-overloaded method works well.