Error when compiling

This was fixed in 1.21, do you get it in 1.21 too, even after recompiling?

Yes, and I tried to remove SF, reset layout and import it again.

That really shouldn't be possible at all, there's nothing that writes gl_ anymore. It sounds like you may have shaders using the same path (a result of duplication), where you've only recompiled one of them. It could also be a cache issue, or a selective recompilation issue, where Unity doesn't recompile the GLSL path because you're not currently using it.

I can try to recompile all my shaders

Try deleting Library/ and Temp/ folders in your project folder first too, to clear the cache

(After closing Unity, then reopen it)

Deleting Library got rid of the Error but now Unity crashes when I compile a shader :/

Edit: Maybe it did not get rid of the eror, I just did not see it because of the Unity crash.

Any errors/crashlog? How does it crash? Does Unity/Shader Forge crash or did the shader just not compile?

Unity crashed. I reinstalled SF again end the error is back

Also got this:

InvalidOperationException: Operation is not valid due to the current state of the object

I'm not quite sure why this happens uniquely to you. That's a GUI error, which pretty much everyone should be getting, including me :(

Maybe it is fighting with my plugins? Using voxelbusters Native plugin and Unity Ads.