Setup - WordPress 3.3 on Windows 2008 R2 server using IIS 7.5
WordPress is setup as a multiuser site with BuddyPress 1.5 installed.
When user A invites user B to be a "user" on/in user A's blog site, WordPress sends an email to user B with details including a link to user A's blog site with an auto generated code appended on the URL path.
When user B clicks the confirmation message they receive a "500 - Internal server error." message in their browser.
WordPress however does accept the code and it does register the new user B into user A's site in the role they configured.
This issue may or may not have something to do with another issue I'm having, here are links to that: