I have some additional information after sudo -u www-data airtime-liquidsoap --debug
Airtime Liquidsoap
DEBUG:root:http://localhost:80/api/get-stream-setting/format/json/api_key/0RXWEPYNYPKI5F7JDITV/
At /usr/local/lib/python2.7/dist-packages/airtime_playout-1.0-py2.7.egg/liquidsoap/ls_lib.liq, line 269, char 25-56:
this value has type
(default:)->
but it should be a subtype of
string (inferred at /usr/local/lib/python2.7/dist-packages/airtime_playout-1.0-py2.7.egg/liquidsoap/ls_lib.liq, line 269, char 5-13)