Reported by customer:

The function LoadPluginExecutable in Runtime/Misc/Plugins.cpp is not thread safe. This causes a problem if two C# threads are simultaneously calling into DLLs for the first time via functions with the DllImport attribute. The problem is with this code:

SetDllDirectoryW(pluginFolder.c_str());
library = LoadLibraryW(pluginFileName.c_str());
loadLibraryError = GetLastError();
SetDllDirectoryW(NULL);

With the right timing, the 2nd call to SetDllDirectoryW can clear out the DLL path after LoadLibraryW has initiated on another thread, causing it to not have the Plugins sub directory as part of the search path.

We encountered this problem in 2018.2.16f1, but looking at the code for 2019.2 it appears to still exist. We've tested locally with a lock around this code and it addresses the problem.