This issue can be caused by different factors, so let's try to apply few solutions to determine the root of the problem:
1. **The User Permissions:** In some cases, the problem can be overcome by explicitly running the command prompt as an administrator. To do so, search for "cmd" in the start menu, right-click on "Command Prompt" and select "Run as administrator".
2. **The PATH environment variable:** The system might not be able to find your program because its directory isn't included in the PATH environment variable. You can verify this by trying to execute your program from another directory. If it doesn't start, then its directory isn't in PATH. To add an absolute path to the PATH variable permanently, use the command: `setx PATH "%PATH%;C:\your\path\here"` – replacing the example path with the path to your program, of course.
3. **The Executable Dependencies:** Your executable may have dependencies on DLLs or other executables that live in the same directory. If you start the program from a different directory or via an absolute path, those dependencies might not be picked up correctly. You would need to make sure any needed DLLs are also within a directory listed in your PATH environment variable.
4. **Improve your command line:** For the purpose of seeing your server in task manager immediately, you can consider changing your command a bit. Instead of using `start OPC-UAServer.exe`, try using `start /B OPC-UAServer.exe`. The `/B` Parameter will start an application without creating a new window.
5. **Task Manager issue:** Sometimes, Task Manager might be having issues refreshing its view. Try to sort processes by name or sort them by PID, also click on "view" within Task Manager and select 'update speed' to high.
Try these steps, see which one helps. If it still doesn’t work, please give more information about the program and your system environment. Also, keep in mind that it's recommended to run such applications not from the command line, but as a Windows service.