[6.0.2]
I was just trying to duplicate this, and was unable to get the "become this user" feature to work at all. It looks like the cookie path isn't correct. Even though I have the cookie path set to "/" in the config settings, the cookies set by admin/loginas.php are using the script path, rather than "/".
Here's a
post on this subject.
Edit:
After fixing admin/loginas.php to use the correct cookie path, the "become this user" feature works correctly (even when the user name contains a space).
Does your problem only occur for users with special characters in their names? If so, then the above may not apply.
