Bug - circular shadow from unknwon source under camera, with size of shadow distance from quality settings

Shadow from unknown source appears under editor / player camera whenever I turn on shadows in my directional light, like something was casting shadow from sky but there is nothing
-Shadow has size of shadow distance from quality settings
-Originally I used unity 5.6.5, tried to migrate the project to 2018 and 2019, but it didn’t help
-Then I tried to create a cean project and move only my asset files into it, bug disappeared for few hours but then appeared again for good
-I use real-time lighting

I found a walkaround solution - I installed NGSS soft shades solution to override Unity shading and the issue seems to be solved. Though I would rather find a way to fix it without instaling .