When type _Form1 is obfuscated, resources connected to this types is also obfuscated(renamed) to be correctly loaded by standard way:
[c#]
ResourceManager rm = new ResourceManager(typeof(_Form1));
// Get the culture of the currently executing thread.
// Retrieve the value of the string resource named "welcome"
String str = rm.GetString("welcome");
use
[c#]
ResourceManager rm = new ResourceManager(typeof(_Form1));
[VB.Net]
Dim rm as ResourceManager = New ResourceManager(Me.GetType());
instead of
[VB.Net]
Dim rm as ResourceManager = new ResourceManager("WindowsApplication._Form1", Me.GetType.Assembly);