I'm getting repeated 500 errors from a lot of Milo requests:
10:24:41: ERROR: Unable to GetBuildbotBuildJSON chromeos kevin-paladin:1178: Got a 500 response with url: https://luci-milo.appspot.com/prpc/milo.Buildbot/GetBuildbotBuildJSON
content: Internal Server Error

10:24:41: INFO: Running cidb query on pid 52662, repr(query) starts with 'SELECT c.id, b.id, action, c.reason, build_config, change_number, patch_number, change_source, time
10:24:42: ERROR: Unable to get suites for chromeos kevin-paladin:1178: Got a 500 response with url: https://luci-milo.appspot.com/prpc/milo.BuildInfo/Get
content: Internal Server Error

If I do the query via the RPC explorer it seems to work:
https://luci-milo.appspot.com/rpcexplorer/services/milo.BuildInfo/Get?request={%20%20%20%20%22buildbot%22:%20{%20%20%20%20%20%20%20%20%22masterName%22:%20%22chromeos%22,%20%20%20%20%20%20%20%20%22builderName%22:%20%22kevin-paladin%22,%20%20%20%20%20%20%20%20%22buildNumber%22:%201178%20%20%20%20}}

And I've been using the same code and having it work for many other requests.