Blank squares when painting landscape

Hey, so yesterday I bought a new pack for UE4. This one - Procedural Nature Pack Vol.1 in Environments - UE Marketplace
Yesterday it was working fine, except for the puddles being just black hole but that’s not the point. When I was creating the landscape yesterday everything was alright. Today I woke up, open UE4, started a new level and when I was trying to create a landscape some weird white squares started flashing. When I create a landscape myself (like creating a new material and then assigning different textures to it) everything is ok.
Here’s a picture of the problem - Imgur: The magic of the Internet
So they keep on flashing and appearing in different areas (where landscape is painted). This has nothing to do with lighting.
Thanks in advance.