Parse error - invalid json-rpc request at configure during bootstrap

Apache 2, Ubuntu 18.04 AWS EC2.
Install foswiki 2.1.6. Visit configure the first time and have red bubble with error

Error

Parse error - invalid json-rpc request: JSON text must be an object or array (but found number, string, true, false or null, use allow_nonref to allow this)

From browser console:

request:

curl 'http://my.url/bin/jsonrpc/configure' -H 'Connection: keep-alive' -H 'Accept: application/json, text/javascript, /; q=0.01' -H 'Origin: http://my.url' -H 'X-Requested-With: XMLHttpRequest' -H 'User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/80.0.3987.87 Chrome/80.0.3987.87 Safari/537.36' -H 'Content-Type: application/json' -H 'Referer: http://my.url/bin/configure?VIEWPATH=' -H 'Accept-Language: ru-RU,ru;q=0.9,en-US;q=0.8,en;q=0.7' -H 'Cookie: FOSWIKI_UPDATESPLUGIN=; FOSWIKISID=9ec79a5f10626fb8d2a55cd40f03317a; FOSWIKISTRIKEONE=1b9b1da00de57d79d6cf4f642d33fadd' --data-binary '{"jsonrpc":"2.0","method":"getspec","params":{"get":{"parent":{"depth":0}},"depth":0},"id":"iLoad-schema_0"}' --compressed --insecure

response:

{"jsonrpc":"2.0","error":{"message":"Parse error - invalid json-rpc request: JSON text must be an object or array (but found number, string, true, false or null, use allow_nonref to allow this)","code":-32700}}

-- PaulTanaskov - 28 Feb 2020