Hi,
We are getting a DllNotFoundException on some Windows machines for a 3rd-party native plugin. But the exact reason is not known. I wasn’t able to find anything Unity-specifc so far. I found related documentation at DllNotFoundException | Mono. As we use Mono runtime, it might be just the tool for us. But the question is, is this compatible with Unity apps? Is there another way to debug this?
What DLL? What plugin?
What’s the location/path to the DLL?
What’s the actual error message?
Don’t leave us in the dark if you want help. 
Perhaps with Process Explorer??? I haven’t used it in a decade or more but I seem to remember there was a way to illustrate what DLLs were being mounted.
All native libraries are at /_Data/Plugins/x86_64. The same build works on other Windows x64 machines that we tested.
Well, as others have said this is not really enough information to give any advice. Since you said it works on some machines but not on others, do you actually have any information on those machines? Hardware, architecture (32 / 64 bit), OS version (Win7/8/10/11 32/64bit), etc ?
Sorry, it’s Windows 10 x64.
And what is the missing DLL(s)?
That we didn’t know.
In the end, we used the tool at GitHub - lucasg/Dependencies: A rewrite of the old legacy software "depends.exe" in C# for Windows devs to troubleshoot dll load dependencies issues., the plugin required MS Visual C++.