Well, that was quick, but thank God, I figured it out.
In my case, I'm programming a Stream Deck button to automatically paste the Textsoap-cleaned text at the cursor. I think the problem was in the AppleScript syntax, because this works well.
I have no idea why the option to "Display trimmed results without errors in a window" works whereas the more intuitive "Save to System Clipboard" does not. Perhaps a bug with Keyboard Maestro? No window is being opened at this action.