How to make the 'Module Updates' report available again?
Here is a list of items to check:


Make sure core module update (= Update Manager) is enabled.
Clear all caches: if you get a WSOD while doing so, it indicates memory shortage.
Try running update.php: if you get a Fatal error like "Allow memory size of ... bytes exhausted", it indicates memory shortage.
Verify if it is a memory shortage issue: try increasing the memory a bit to check if there is any difference.