rfacebook http 500 error

hey, here's my problem:
I have a working rails application and everytime I install the
rfacebook plugin (nothing more than running script/pluin install) all
I get from the app is http 500s.
I also did rake facebook:setup and it didnt help.
Nothing goes to the logs file so I have no idea where to start...
and of course once I delete the plugin dir the app comes back to life.

anyone ?
thnx alot...