Moving this to Game Help boards~
Additionally, check to make sure all colors in your terrain mask are correct~
When making a terrain mask, you need to make sure all base colors, red, green and blue, are "hard" colors.
(https://tinyurl.com/w4rlle7)
The numbers circled need to be at 0 on all colors (for red) when applying them to the mask. Otherwise, it'll crash and or just not work period. So if you need red, blue and green will be 0. If you need blue, green and red will be 0. So on and so forth.
(https://tinyurl.com/rzzowat)
^ Red ^
(https://tinyurl.com/t39vsyp)
^ Green ^
(https://tinyurl.com/ujqv3op)
^ Blue ^
So now that you're working with "hard" RGB, you can of course blur them together as much as you'd like.
Hope this helps~
Glad you got it figured out! I'll be locking this since your issue is resolved =)