How do I fix "Error 400 on SERVER: (<unknown>): did not find expected key while parsing a block mapping at"?

edit

The issue appears to be that I wasn't quoting a variable substitution in a YAML map. E.g. this is wrong:

myclass::mymap:
  firstkey: firstvalue
  mykey: %{::myvariable}.domain.tld
  myotherkey: othervalue

While it should be

myclass::mymap:
  firstkey: firstvalue
  mykey: "%{::myvariable}.domain.tld"
  myotherkey: othervalue

It appeared that the f of firstkey was on line 19 column 3. So, be aware that the error does not always indicate the right line.