I wanted to check this in the forum before I submitted a formal bug report.
The “+” (plus) character seems to cause problems in the blog URL.
For example, I formerly had a blog located at http://www.busterandfriends.com/io++ (sorry, it’s no longer located there). This caused various (buggy .htaccess-like) anomalous behavior. For example, only the default permalink structure would work, any other would result in a 404. (I was able to reproduce the bug while running the WP default theme, and after disabling all plugins.)
I’ve subsequently moved the blog to http://www.busterandfriends.com/io which solved all problems.
BTW, I’m not proposing that there should be no illegal characters in the blog address, but if there are such characters, it would be handy if they were signposted during installation.
Theme: Sandbox 1.6 variant
Plugins: tried with both Akismet only, and no plugins