In order, as described in the wiki:

Arbiter registers all objects in memcached
Scheduler looks up the objects in memcaches
Poller looks up the objects in memcached and does collection
Poller updates the objects in memcached and returns check results

Each daemon always calls the same snmp_poller.py file. So you can debug everything in there. You can also start the arbiter manually using :
cd /usr/local/shinken
./bin/shinken-arbiter

You can then scroll to see if you have any errors and where they happen.

Did you find what what file was causing the file error in your configuration?

cheers,

xkilian