I get an error:

Error 1001. Exception occurred while initializing the installation:
System IO.FileNotFoundException: Could not load file or assembly 'file:///C:\WINDOWS\system32\Rountree' or one of its dependancies. The system could not find the file specified.

Rountree is my surname so Im assuming its looking for a directory? with my full name. I am set up as a user on the system using my full name that has a space in it. I selected install for "Everyone" during the install. I get the same error when selecting "Just Me"

Yes, we have added a custom installer action to the 5.1.1 installer, and it takes the account's login name as an argument. Now we've found this error happens when the login name contains space characters, but don't know why yet. As a workaround you can install it from a different account that doesn't contain spaces in its name, e.g. Administrator.

I hope that helps,
Stoyan