In some cases (which I'm still not 100% clear on), the post-review cookies can get messed up.  This causes post-review to go into a recursive loop trying to log in.  This is a simple fix which appears to solve the problem.

Using a cookie file that I could reproduce the problem with, I verified that it asked for my login after failing to post using the existing cookie.  I also verified that with a good cookie it still works, and with no cookies it asks for your login as expected.

I imagine you could reproduce this by editing post-review-cookies.txt and mangling the rbsessionid.