Texture artifacts - Unity 2018.4.8f1 LWRP

Hello,

As I'm trying to develop an AR app while not being a long time user, I've faced several problems:

- background camera all black, which led me to downgrade to 2018.4.8f1 more workable version.
- very bad results with lightmaps even by tweaking settings up and down, which led me unwillingly to use real-time lighting.
- texture artifacts in the app (not in the Unity scene though) noticeable on the walls in this picture:



As anybody a hint on what might cause this artifacts??
(also if there is some explaination on what can cause very bad lightmaps I'd be glad to hear )

Regards-