FFB sources with 0 t/yr on FFB contribution are usual suspects in this problem.
The current system doesn't take zero values well, and we are actively working to resolve this issue for the long term when PalmGHG v5 is implemented.
However, in the meantime, we would recommend revising these to non-zero values, and also checking that the fertilizer tab found in each of the FFB sources is correctly recorded.
One thing that may help is checking that each FFB source shows a non-zero table Summary tab.
This can be done by using a non-zero value for FFB supplied and a fertilizer value that is >0.01 t/yr as that tends to be rounded to 0.
Please contact ghg@rspo.org if your issues with the Summary(Mill) tab persist after attempting this fix.