Extract from the logs:

Hostgroup Load (0.9ms)  SELECT DISTINCT puppetclasses.id FROM "hostgroups" INNER JOIN "hosts" ON "hosts"."hostgroup_id" = "hostgroups"."id" INNER JOIN "hostgroups_puppetclasses" ON "hostgroups_puppetclasses"."hostgroup_id" = "hostgroups"."id" INNER JOIN "puppetclasses" ON "puppetclasses"."id" = "hostgroups_puppetclasses"."puppetclass_id" WHERE (hosts.name = 'somehost')
  Puppetclass Load (0.6ms)  SELECT name, id FROM "puppetclasses" WHERE ((puppetclasses.id = 'nil'))
PG::Error: ERROR:  invalid input syntax for integer: "nil" 
LINE 1: ..., id FROM "puppetclasses" WHERE ((puppetclasses.id = 'nil'))
                                                                ^
: SELECT name, id FROM "puppetclasses" WHERE ((puppetclasses.id = 'nil'))
Operation FAILED: PG::Error: ERROR:  invalid input syntax for integer: "nil" 
LINE 1: ..., id FROM "puppetclasses" WHERE ((puppetclasses.id = 'nil'))
                                                                ^
: SELECT name, id FROM "puppetclasses" WHERE ((puppetclasses.id = 'nil'))
app/controllers/puppetclasses_controller.rb:22:in `index'
app/controllers/puppetclasses_controller.rb:17:in `index'
Completed 500 Internal Server Error in 25ms

Suggested fix is to change in app/models/puppetclass.rb the condition "puppetclasses.id = 'nil'" to "1=0" to produce a valid false statement.
There's maybe a more portable way of producing a false statement, but I know for sure that sqlite doesn't support "false".