I created Extension, but any attempt to call ANY STATIC FUNCTION from extenstion, or use ANY STRUCT/CLASS e.T.c brings unknown plugin compilation error
Here is extension overview:
Plugin:

Error looks like error...????

Even function getOne() which just returns "1" gives error.

However, from entire extension, all methods calls correctly.

How to fix it?

ServerReporterAPIClient.getOne()

According to the above code, you are missing the t in Client.

Wolf, this plugin is just pseudocode, which i wrote to demonstrate trouble to u, problem not in this)
I just made a mistake in class name 

Are you sure the extension is fully loaded?

Yes, another thread of extension works correctly, i just even tryed to use //Reference: mydll.dll is the begining of plugin- no affect.

I found in compilator log something like this NBut my plugin wrote on .net core 2.0, and dnspy confirms that

Merged post

Okay i fixed this. 
I changed target version from .net core 2.0 to .net framework 4.7.2.

Thanks anyway)

Right, extensions in Rust would only work with the framework it uses and lower. .NET Standard 2.0 or lower would be fine. For universal extensions, .NET 3.5 should be used.