The likely underlying cause is that the nonce is getting truncated when the number of fields or parameters exceeds PHP’s max_input_vars setting.
My guess is that @clicknathan’s fix works for those close to exceeding the limit, as disabling the metabox provides some needed leeway in the total number of input variables.
For us, the fix seems to be increasing the site’s or server’s max_input_vars setting to 10,000, depending on the environment.
