It seems like OpenID Provider (OIDP) is incompatible with the quite-popular Persistent Login (PL). I use PL and OIDP together on my site, and whenever I try to log in to a relying site with a new browser session, I end up with a blank white screen at $base_url/openid/provider. If I then try to log in a second time, everything works fine, which leads me to believe that there's a conflict with Persistent Login.