API unavailable

We have recently encountered an issue where the API suddenly becomes unavailable.

How do I begin to troubleshoot this?

Well, the error indicates, that httpd receives an invalid response from the Rails Application

Does the UI still work?
What happens, if you do a local curl? curl http://localhost:3000/api
Does the problem persist after restarts (application and VM)?
Are there any hints in vmdb/log/evm.log or in the apache logs?

Which version of CloudForms or ManageIQ? This was recently fixed with https://github.com/ManageIQ/manageiq-api/commit/abd070af52cebc5a4707fe3c2b1c3db2a350f6c5 and the CFME 5.9.4 update I believe.