Can't install ISA Server if your username contains a hash (#)
This is one of the stranger issues I've had to troubleshoot. Customer decided they were going to identify their administrative users with a # symbol (which I shall call a hash, rather than repeatedly trying to type octothorpe without laughing out loud).
Here's what happens if your administrative user happens to be called "#djr".
- Run Setup.
- Install your desired selection of components, include Enhanced Logging (this is where the issue lies).
- Core Components installs successfully

- Error:

- Try again. Same error.
- Reboot, try again. Same error.
- Search Google for the error. Nothing.
- Scratch head
- Notice that the hash in the username has been replaced with the folder path to which SQL binaries will be installed.
So it seems that the ISA installer uses the hash internally as a placeholder for something, and the use in the username causes it some confusion.
Renamed the account, installed without error straight away.