undefined method `call' for :notifo:Symbol

I have a rails app which is running rails 3.2.22 on Ruby 2.2.5 on Heroku.

I am getting an error which I am having trouble diagnosing;

NoMethodError (undefined method `call' for :notifo:Symbol):

This is the stack trace for it:

vendor/bundle/ruby/2.2.0/gems/activesupport-3.2.22/lib/active_support/deprecation/reporting.rb:13:in `block (2 levels) in warn'
vendor/bundle/ruby/2.2.0/gems/activesupport-3.2.22/lib/active_support/deprecation/reporting.rb:13:in `each'
vendor/bundle/ruby/2.2.0/gems/activesupport-3.2.22/lib/active_support/deprecation/reporting.rb:13:in `block in warn'
vendor/bundle/ruby/2.2.0/gems/activesupport-3.2.22/lib/active_support/deprecation/reporting.rb:12:in `tap'
vendor/bundle/ruby/2.2.0/gems/activesupport-3.2.22/lib/active_support/deprecation/reporting.rb:12:in `warn'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_pool.rb:212:in `block in clear_stale_cached_connections!'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_pool.rb:210:in `each'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_pool.rb:210:in `clear_stale_cached_connections!'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_pool.rb:274:in `block (2 levels) in checkout'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_pool.rb:242:in `loop'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_pool.rb:242:in `block in checkout'
vendor/ruby-2.2.5/lib/ruby/2.2.0/monitor.rb:211:in `mon_synchronize'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_pool.rb:239:in `checkout'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_pool.rb:102:in `block in connection'
vendor/ruby-2.2.5/lib/ruby/2.2.0/monitor.rb:211:in `mon_synchronize'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_pool.rb:101:in `connection'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_pool.rb:410:in `retrieve_connection'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_specification.rb:171:in `retrieve_connection'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_specification.rb:145:in `connection'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/query_cache.rb:67:in `rescue in call'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/query_cache.rb:61:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/activerecord-3.2.22/lib/active_record/connection_adapters/abstract/connection_pool.rb:479:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/actionpack-3.2.22/lib/action_dispatch/middleware/callbacks.rb:28:in `block in call'
vendor/bundle/ruby/2.2.0/gems/activesupport-3.2.22/lib/active_support/callbacks.rb:405:in `_run__1702427010739121446__call__964470083021375166__callbacks'
vendor/bundle/ruby/2.2.0/gems/activesupport-3.2.22/lib/active_support/callbacks.rb:405:in `__run_callback'
vendor/bundle/ruby/2.2.0/gems/activesupport-3.2.22/lib/active_support/callbacks.rb:385:in `_run_call_callbacks'
vendor/bundle/ruby/2.2.0/gems/activesupport-3.2.22/lib/active_support/callbacks.rb:81:in `run_callbacks'
vendor/bundle/ruby/2.2.0/gems/actionpack-3.2.22/lib/action_dispatch/middleware/callbacks.rb:27:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/actionpack-3.2.22/lib/action_dispatch/middleware/remote_ip.rb:31:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/actionpack-3.2.22/lib/action_dispatch/middleware/debug_exceptions.rb:16:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/actionpack-3.2.22/lib/action_dispatch/middleware/show_exceptions.rb:56:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/railties-3.2.22/lib/rails/rack/logger.rb:32:in `call_app'
vendor/bundle/ruby/2.2.0/gems/railties-3.2.22/lib/rails/rack/logger.rb:16:in `block in call'
vendor/bundle/ruby/2.2.0/gems/activesupport-3.2.22/lib/active_support/tagged_logging.rb:22:in `tagged'
vendor/bundle/ruby/2.2.0/gems/railties-3.2.22/lib/rails/rack/logger.rb:16:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/request_store-1.3.2/lib/request_store/middleware.rb:9:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/actionpack-3.2.22/lib/action_dispatch/middleware/request_id.rb:22:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/rack-1.4.7/lib/rack/methodoverride.rb:21:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/rack-1.4.7/lib/rack/runtime.rb:17:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/rack-1.4.7/lib/rack/lock.rb:15:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/actionpack-3.2.22/lib/action_dispatch/middleware/static.rb:83:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/rack-ssl-1.3.4/lib/rack/ssl.rb:27:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/rack-cache-1.7.1/lib/rack/cache/context.rb:140:in `forward'
vendor/bundle/ruby/2.2.0/gems/rack-cache-1.7.1/lib/rack/cache/context.rb:249:in `fetch'
vendor/bundle/ruby/2.2.0/gems/rack-cache-1.7.1/lib/rack/cache/context.rb:189:in `lookup'
vendor/bundle/ruby/2.2.0/gems/rack-cache-1.7.1/lib/rack/cache/context.rb:66:in `call!'
vendor/bundle/ruby/2.2.0/gems/rack-cache-1.7.1/lib/rack/cache/context.rb:51:in `call'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/railties-3.2.22/lib/rails/engine.rb:484:in `call'
vendor/bundle/ruby/2.2.0/gems/railties-3.2.22/lib/rails/application.rb:231:in `call'
vendor/bundle/ruby/2.2.0/gems/railties-3.2.22/lib/rails/railtie/configurable.rb:30:in `method_missing'
vendor/bundle/ruby/2.2.0/gems/newrelic_rpm-3.9.4.245/lib/new_relic/agent/instrumentation/middleware_tracing.rb:57:in `call'
vendor/bundle/ruby/2.2.0/gems/puma-3.11.0/lib/puma/configuration.rb:225:in `call'
vendor/bundle/ruby/2.2.0/gems/puma-3.11.0/lib/puma/server.rb:624:in `handle_request'
vendor/bundle/ruby/2.2.0/gems/puma-3.11.0/lib/puma/server.rb:438:in `process_client'
vendor/bundle/ruby/2.2.0/gems/puma-3.11.0/lib/puma/server.rb:302:in `block in run'
vendor/bundle/ruby/2.2.0/gems/puma-3.11.0/lib/puma/thread_pool.rb:120:in `call'
vendor/bundle/ruby/2.2.0/gems/puma-3.11.0/lib/puma/thread_pool.rb:120:in `block in spawn_thread'

To start with the app functions normally, but then after a bunch of requests are made in quick succession this error crops up and then all subsequent requests (which had been working fine before) throw this error and the app crashes.

I can't find any reference to this particular error and I'm rather stumped. The app does not appear to be hitting heroku memory limits so I don't think that is the cause. Could this be a rails 3 issue, or the result of upgrading to rails 3 (from 2)?

Any help on this would be greatly appreciated! thanks.