I had a similar problem with 5212 - there appears to be some problem with the following file. Or it could be something intentional going on that I’m just not bright enough to figure out.
On line #44 of vendor/rails/actionpack/lib/action_controller/templates/rescues/_request_and_response.rhtml
41: 42: 43: <h2 style="margin-top: 30px">Response</h2> 44: <b>Headers</b>: <%=h response.headers.inspect.gsub(/,/, ",\n") %><br/>
I was able to do a temporary hack to get my error screens back by changing line 44 to the following:
Headers: <%=h response.headers.inspect.gsub(/,/, “,\n”) if response and response.headers
Hope this helps.