Random(?) Timeout and 500 Service Error

[etanx]

This is my 2nd time trying out a Slack Wolfram bot! First time was a clunky Python's slack-client and wolframalpha packages. Followed the tutorial, uploaded code to GitHub an deployed with Heroku. Dynos start up fine when I checked the with heroku ps, and logs show that build succeeded. But when testing the slash command Slackbot will randomly give an error (depending on the query), e.g.:

Darn - that slash command didn't work (error message: Timeout was reached).
Darn - that slash command didn't work (error message: 500_service_error).

Heroku logs would look something like this:
at=info method=POST path="/wolfa" host=wolfa.herokuapp.com request_id=5a9a5151-fdd1-4b15-94a8-4263a1b1c1ab fwd="34.234.74.21" dyno=web.1 connect=1ms service=2186ms status=500 bytes=450 protocol=https2019-02-20T18:55:43.141647+00:00 app[web.1]: 10.5.162.108 - - [20/Feb/2019 18:55:43] "POST /wolfa HTTP/1.1" 500 -

or this

at=info method=POST path="/wolfa" host=wolfa.herokuapp.com request_id=a740f3f6-51c2-47ad-bb13-56bf4edb1bcd fwd="34.239.254.200" dyno=web.1 connect=0ms service=2257ms status=500 bytes=450 protocol=https

It's quite puzzling, since I can ask Wolfram '2+2' and 'earth speed' but 'light speed' and 'inches' doesn't work. Any ideas?