I'm also encountering this issue. It specifically occurs when Unity tries to serialize a NetworkVariable<FixedString32Bytes>. I am able to set the value of the network variable and read it again ...

Worse, it got moved to its own assembly - literally a DLL which only contains this class, which means there is no type we can use to get at its location. So, sigh, we'll need to add yet another ...

Ошибка в Unity NullReferenceException: Object reference not set to an instance of an object ...