Sending keypresses to another app/window

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C#

Windows

.NET

I have an application which sends keypresses to another application window using this code:

ADR_Library.DataAccess.Win32Imports.SetForegroundWindow(handle);
System.Windows.Forms.SendKeys.SendWait(item.Key);
System.Windows.Forms.SendKeys.Flush();
System.Threading.Thread.Sleep(1000);

This was working fine but in the last week it has stopped working. An older release of the programme has stopped working too. I know that the code finds the correct 'handle' for the other window, but the data does not appear in the destination window. Why? This works on some other PCs, but not mine anymore.

This sounds to me like some sort of Windows permissions issue, as if a Windows Update has perhaps changed something.

What I have tried:

Running my application under the Admin account does not solve the problem.