I installed FLUKA and Flair on window10 using WSL (ubuntu 18.04). I tried to load my previous flair project and it works well, but there is an error for a 2-D color fluence plot as the following figure shows
I the latest Flair version, there was an overhaul of the color palettes.
It seems that you selected the color bar manually in your plot, and it is not working any more. You may try to select it again, or simply recreate the plot.