Thanks for the help and the advice!
Heterozygosity is indeed low (that was one of the criteria to select the sample for sequencing). So if I understand correctly, the diploid model requires a substantial level of heterozygosity to work. It may be good to mention that requirement in the documentation.
I will use the haploid model except for samples with higher levels of heterozygosity.
Can you please send both HTML reports to kmergenie@cse.psu.edu?
The diploid model is more constrained, so it has higher chance to not fit to an histogram, as opposed to the haploid model, that is less constrained.