It is for usernames with spaces. I've documented it at 4 sites. I haven't been able to get it with a user without spaces except when an image is using a space (%20) :/...
This is fixed in 6.7.2. Names with spaces have nothing to do with it. <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />
For record keeping purposes, the fix landed in rv210.2.11.