I get the same issue but only with Giant size (or bigger) maps. Enormous (128x80) size works fine for me, which me makes me think that it's not an issue with the YnAMP mod but maybe system RAM, or the way WorldBuilder is coded? I have 16 GB of RAM and I haven't had any issues with Enormous.
When you say "enormous size map" do you literally mean the "Enormous (128x80)" size or just a "big" map? Enormous consistently works fine for me, but I'm unable to select plots in any of the bigger map sizes.