on the client machines (the "secret sauce" for many RT fixes). He re-registered the files manually.
Installing SIMATIC NET V16 solved the problem, both WinCC Viewer RT and IE web client connexions work now. Siemens SiePortal Web Navigator problem - SiePortal wincc viewer rt download fix
Open the WinCC Viewer RT configuration application on the client PC. on the client machines (the "secret sauce" for