Hey vagrantart,
I am assuming when you say ‘procedural vegetation’ you are using the LandscapeGrassType node within the material editor to define where the foliage is placed, in combination with the foliage paint tool to hand paint the foliage as well?
I want to try and reproduce your crash on my end, so gathering all the accurate information to get the crash to occur is going to be really helpful. For a more accurate test, which might not be the best option since it sounds like your project is rather large, you can zip up your exisitng project and share it with me via a private message after uploading it a shareable drive like Google Drive or Dropboxx.
You mention that a potential clue to the crash occurring is your landscape painting tools stopped working. If you could get me to this point, that might allow me to get the crash to occur on my end as well.
Thank you,