I was working on a WSUS server that was ready to complete its post-installation tasks. When I clicked Run...
...the post-installation tasks failed with Fatal Error: Illegal characters in path.
Looking at the log file referenced in the error was not helpful. The issue ended up being the trailing backslash in the path. After removing the backslash and clicking Run again, the post-installation tasks completed successfully.
It seems silly to me that Microsoft hasn't fixed this, but at least it's easy to work around once you figure out what the problem is.