Thanks to (gile) who gives us the solution:
To launch DLL from a network drive, you have to authorise that on each computer.
One way to do this is to open a command window on each computer, go to .NET install directory (generaly "c:\windows\microsoft.net\framework\version\") , and enter the following command (with the true path to installed DLLs) :
caspol.exe -machine -quiet -addgroup 1 -url "file://Server/pathtoDLL/*" FullTrust