I have been unable to pinpoint the precise set of conditions which causes this to occur, but I have a general idea:

If I render the Expense Piechart with 'Maximum Slices' set to X, it renders as expected. However, if I set 'Maximum Slices' to X+1, it simply fails to render (the entire tab remains white, no text or charts, error messages, or even artifacts of any kind). The value of X varies depending on the number of accounts being shown. By de-selecting accounts from being included in the report, I can decrease the value of X necessary to trigger this behavior.

While playing with this, I have never observed the 'Other' category dip below 30% on a chart which renders. I suspect (but have no evidence to back my suspicion up) that the handling of this 'Other' category, or the boundary conditions surrounding it, are the culprit.

The above observations were made with 'Sort Method' set to 'Amount', though the behavior is similar with the other sort methods. All other report options were set to the defaults except for those explicitly mentioned.