Try force closing the Mail app: from the home screen double tap the home button, locate Mail in the list at the bottom, tap and hold it until it wiggles, tap to close it, tap the home button again. You could also try going to Settings>iCloud and turning Mail to Off, then back to On.
If it persists, you may have to give it some time for the additional storage to register. Otherwise you'll have to contact iCloud support.
I was able to solve the problem by syncing my iphone 4s thru Icloud.
I did back up my iphone thru Itunes and tried to restore thru Itunes but it cannot complete the restoration due
to ".........corrupt data".
Having an Icloud back-up is essential and should help you restore seamlessly thru a wifi connection.
I was then able to restore the important contacts list and my calendar (thru Icloud), after having some problems with the Itunes restoration - it just cannot complete the restoration after a back-up.
I lost the SMS messages and my email data and settings.
Hope this can help others with this problem with their Calendar.......