This is pure guess, but under Status -> RRD Graphs -> Settings, there's a "Enables the RRD graphing backend." check box.
I'd try un-checking it, saving setting, re-enabling it, save settings.
This is kind of a "dumb GUI trick", but things like this sometimes work with GUI's.
I have a bit of trouble finding where all of the logs are kept, but checking them is never a bad idea.