About your application's environment
Ruby version 1.8.7 (x86_64-linux)
RubyGems version 1.3.7
Rack version 1.1
Rails version 2.3.5
Active Record version 2.3.5
Active Resource version 2.3.5
Action Mailer version 2.3.5
Active Support version 2.3.5
Edge Rails revision unknown
Application root /usr/share/redmine
Environment production
Database adapter postgresql
Database schema version 20110201111915

About your Redmine plugins
Redmine Opensearch plugin 0.1
Redcase 1.0 alpha 0.144
Redmine Google Docs Plugin plugin 0.0.1
Redmine Graphs plugin 0.1.0

svn, version 1.6.12 (r955767)

production.log:

Processing RepositoriesController#show (for 192.168.0.19 at 2011-03-16 17:53:24) [GET]
Parameters: {"action"=>"show", "id"=>"support", "controller"=>"repositories"}
NoMethodError (undefined method left' for nil:NilClass):
/usr/lib/ruby/1.8/active_record/associations/association_proxy.rb:217:insend'
/usr/lib/ruby/1.8/active_record/associations/association_proxy.rb:217:in method_missing'
app/models/changeset.rb:186:infind_referenced_issue_by_id'
app/models/changeset.rb:125:in scan_comment_for_issue_ids'
app/models/changeset.rb:124:ineach'
app/models/changeset.rb:124:in scan_comment_for_issue_ids'
app/models/changeset.rb:120:inscan'
app/models/changeset.rb:120:in scan_comment_for_issue_ids'
app/models/changeset.rb:95:inafter_create'
/usr/lib/ruby/1.8/active_record/callbacks.rb:347:in send'
/usr/lib/ruby/1.8/active_record/callbacks.rb:347:incallback'
/usr/lib/ruby/1.8/active_record/callbacks.rb:267:in create'
/usr/lib/ruby/1.8/active_record/base.rb:2874:increate_or_update_without_callbacks'
/usr/lib/ruby/1.8/active_record/callbacks.rb:250:in create_or_update'
/usr/lib/ruby/1.8/active_record/base.rb:2538:insave_without_validation'
/usr/lib/ruby/1.8/active_record/validations.rb:1078:in save_without_dirty'
/usr/lib/ruby/1.8/active_record/dirty.rb:79:insave_without_transactions'
/usr/lib/ruby/1.8/active_record/transactions.rb:229:in send'
/usr/lib/ruby/1.8/active_record/transactions.rb:229:inwith_transaction_returning_status'
/usr/lib/ruby/1.8/active_record/connection_adapters/abstract/database_statements.rb:136:in transaction'
/usr/lib/ruby/1.8/active_record/transactions.rb:182:intransaction'
/usr/lib/ruby/1.8/active_record/transactions.rb:228:in with_transaction_returning_status'
/usr/lib/ruby/1.8/active_record/transactions.rb:196:insave'
/usr/lib/ruby/1.8/active_record/transactions.rb:208:in rollback_active_record_state!'
/usr/lib/ruby/1.8/active_record/transactions.rb:196:insave'
/usr/lib/ruby/1.8/active_record/base.rb:723:in create'
app/models/repository/subversion.rb:59:infetch_changesets'
/usr/lib/ruby/1.8/active_record/connection_adapters/abstract/database_statements.rb:136:in transaction'
/usr/lib/ruby/1.8/active_record/transactions.rb:182:intransaction'
/usr/lib/ruby/1.8/active_record/transactions.rb:188:in transaction'
app/models/repository/subversion.rb:58:infetch_changesets'
app/models/repository/subversion.rb:57:in reverse_each'
app/models/repository/subversion.rb:57:infetch_changesets'
/usr/lib/ruby/1.8/active_record/associations/association_proxy.rb:217:in send'
/usr/lib/ruby/1.8/active_record/associations/association_proxy.rb:217:inmethod_missing'
app/controllers/repositories_controller.rb:77:in show'
/usr/lib/ruby/1.8/action_controller/base.rb:1331:insend'
/usr/lib/ruby/1.8/action_controller/base.rb:1331:in perform_action_without_filters'
/usr/lib/ruby/1.8/action_controller/filters.rb:617:incall_filters'
/usr/lib/ruby/1.8/action_controller/filters.rb:610:in perform_action_without_benchmark'
/usr/lib/ruby/1.8/action_controller/benchmarking.rb:68:inperform_action_without_rescue'
/usr/lib/ruby/1.8/action_controller/benchmarking.rb:68:in perform_action_without_rescue'
/usr/lib/ruby/1.8/action_controller/rescue.rb:160:inperform_action_without_flash'
/usr/lib/ruby/1.8/action_controller/flash.rb:146:in perform_action'
/usr/lib/ruby/1.8/action_controller/base.rb:532:insend'
/usr/lib/ruby/1.8/action_controller/base.rb:532:in process_without_filters'
/usr/lib/ruby/1.8/action_controller/filters.rb:606:inprocess'
/usr/lib/ruby/1.8/action_controller/base.rb:391:in process'
/usr/lib/ruby/1.8/action_controller/base.rb:386:incall'
/usr/lib/ruby/1.8/action_controller/routing/route_set.rb:437:in call'
/usr/lib/ruby/1.8/action_controller/dispatcher.rb:87:indispatch'
/usr/lib/ruby/1.8/action_controller/dispatcher.rb:121:in _call'
/usr/lib/ruby/1.8/action_controller/dispatcher.rb:130:inbuild_middleware_stack'
/usr/lib/ruby/1.8/active_record/query_cache.rb:29:in call'
/usr/lib/ruby/1.8/active_record/query_cache.rb:29:incall'
/usr/lib/ruby/1.8/active_record/connection_adapters/abstract/query_cache.rb:34:in cache'
/usr/lib/ruby/1.8/active_record/query_cache.rb:9:incache'
/usr/lib/ruby/1.8/active_record/query_cache.rb:28:in call'
/usr/lib/ruby/1.8/active_record/connection_adapters/abstract/connection_pool.rb:361:incall'
/usr/lib/ruby/1.8/action_controller/string_coercion.rb:25:in call'
/usr/lib/ruby/1.8/rack/head.rb:9:incall'
/usr/lib/ruby/1.8/rack/methodoverride.rb:24:in call'
/usr/lib/ruby/1.8/action_controller/params_parser.rb:15:incall'
/usr/lib/ruby/1.8/action_controller/session/cookie_store.rb:93:in call'
/usr/lib/ruby/1.8/action_controller/failsafe.rb:26:incall'
/usr/lib/ruby/1.8/rack/lock.rb:11:in call'
/usr/lib/ruby/1.8/rack/lock.rb:11:insynchronize'
/usr/lib/ruby/1.8/rack/lock.rb:11:in call'
/usr/lib/ruby/1.8/action_controller/dispatcher.rb:106:incall'
/usr/lib/ruby/1.8/phusion_passenger/rack/request_handler.rb:92:in process_request'
/usr/lib/ruby/1.8/phusion_passenger/abstract_request_handler.rb:207:inmain_loop'
/usr/lib/ruby/1.8/phusion_passenger/railz/application_spawner.rb:418:in start_request_handler'
/usr/lib/ruby/1.8/phusion_passenger/railz/application_spawner.rb:418:instart_request_handler'
/usr/lib/ruby/1.8/phusion_passenger/railz/application_spawner.rb:358:in handle_spawn_application'
/usr/lib/ruby/1.8/phusion_passenger/utils.rb:184:insafe_fork'
/usr/lib/ruby/1.8/phusion_passenger/railz/application_spawner.rb:354:in handle_spawn_application'
/usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:352:insend'
/usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:352:in main_loop'
/usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:196:instart_synchronously'
/usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:163:in start'
/usr/lib/ruby/1.8/phusion_passenger/railz/application_spawner.rb:213:instart'
/usr/lib/ruby/1.8/phusion_passenger/spawn_manager.rb:262:in spawn_rails_application'
/usr/lib/ruby/1.8/phusion_passenger/abstract_server_collection.rb:126:inlookup_or_add'
/usr/lib/ruby/1.8/phusion_passenger/spawn_manager.rb:256:in spawn_rails_application'
/usr/lib/ruby/1.8/phusion_passenger/abstract_server_collection.rb:80:insynchronize'
/usr/lib/ruby/1.8/phusion_passenger/abstract_server_collection.rb:79:in synchronize'
/usr/lib/ruby/1.8/phusion_passenger/spawn_manager.rb:255:inspawn_rails_application'
/usr/lib/ruby/1.8/phusion_passenger/spawn_manager.rb:154:in spawn_application'
/usr/lib/ruby/1.8/phusion_passenger/spawn_manager.rb:287:inhandle_spawn_application'
/usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:352:in __send__'
/usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:352:inmain_loop'
/usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:196:in `start_synchronously'
/usr/lib/phusion_passenger/passenger-spawn-server:61

Rendering /usr/share/redmine/public/500.html (500 Internal Server Error)

The symptoms are the same as described here: http://www.redmine.org/boards/2/topics/16046
The patch referenced in that post fixes the problem for me.