External Plug-In Unknown Command Error
While the possibility is low that your plug-in package may have error that prevent your app being automatically loaded, I'd say, it is still possible. So, it is up to you to make it certain.
You would simply try to load it manually with command "NETLOAD" and then try your commands in the app. If it still gives the "Unknown command", you now know it is the app itself has problem, not the auto-load package. Considering it worked with your own development computer, the first thing you need to look at the code of the app is whether the app implements IExtensionApplication. If yes, is the code in Initialize() correctly wrapped with try...catch... block? Not caught exception raised in Initailize() would render the loaded assembly (your app) useless, thus "unknown command".
So, its just an entirely third party plug-in and you only updated the auto-load package? Does it come as installation package (*.msi), or you simply downloaded the package and copied/pasted the DLL or DLLs in place? Are there any other dependent DLLs the app uses located in a network location/folder?
My guess is that you just copied the downloaded DLL(s) to the place. Since Windows 7 (AFAIK), Windows would flag any executable file (EXE, or DLL...) that is downloaded from the internet (or even your organization's intranet, sometimes, depending on how the intranet is formed) as unsafe and locks it up as a executable file from network somewhere. That could the cause of the error you saw.
You can go to the DLL file(s) and right-click it and select "Properties", in the "General" tab at bottom, you might see "Security: ...." message with a "Unlock" check box. If so, this the reason: the file was from the internet and potentially unsafe. You can tick the check box to unlock it, as long as you know the file is OK to use.
