There was a Domino server happily running on Windows 2008 64-bit and there was a scheduled LS agent too.
Usually that agent, using OLE Automation, created MS Excel worksheets there were temporary saved in the Windows temp folder: suddenly it stopped working! how come?!
It was useless to trap any error message inside the code...No error messages, nothing, and only when running server-side (client-side it was fine).
Before almost getting crazy, the answer to the was found on the IBM forum (thanks Beppe) that send me to the StackOverflow post.
I created the missing folder "Desktop" in the following path:
After that, the agent was running as expected.