I don't know what web server is there.
When I make the same request in different software (i.e. Visual Studio Code), there is also response with code 206, but there is also proper JSON in response body.
EDIT:
I tested it with standard HttpClient in Delphi 10.3.2 and I get proper JSON response. Of course response code is 206
EDIT2:
I used Indy in Lazarus and response is the same like with fphttpclient (only 3 chars)
EDIT3:
I made simple console application in Lazarus to easy compile with different version of fpc. After compiling program with FPC version I use now 3.3.1 rev 43202 I saw in console that response is much longer than 3 chars. It is:
íýGŽ$IŇĽ˙_% kK?t9÷]´g 3˛"#'ČE"P°t7"┼yŔÍ<?DÜ˝ŕîYQžé'?¬* 6?XD¨¨ą«ë#ęfËĐ┼ć'!0B?J/?"JÚa<'Ą┼(!? 'Úp<[%"p3qYBa:'47"ÓŢż4?o"XçJĆÂ]˙îR˙ŃţÄeĆd&Ívn¸cď9t*Mę(_ŻĄÖ2Wü!Â"Ĺ (Ä3/'+\Ą|˙ÄeÖ¨|('"/ŤXl7Uú|1ë˙żZŚ┼ă ČňgˇMÄu:QŮ6Ęu??z'TRŇw"&ä×Âi.ŃKŇű*cRĐč/°c?RRgĆ\§Ú.&;ôÖŇJĹž|[CH=lx!|<ţÁWbĆŤĐ3aĘB%ÂGžiŃčĺC&ăZŞ8+'ÇyÝhňvčC 7<RţGŁÉ|p
ÄÁŽ?ŕtÍŻú?Î?mŰ?vˇěč?XÎcDŘ!'ŔÇ-".&Á>đJÄP7'ćYO'Ŕ¤b-t_%?L¬E]Ń=u[vĎđň,YYž<Y×.H%ďcőué Ř{V"!h+ěűű@
vËÝr÷žÔ¬p @1q;?Ř4ŔížďŐŻí.ďŢfR┼o¨xP`ËEČňĐśe!A°ZóUŐ§E*z>"ß?Âl{R÷Tnz<6ňITĚň¤Ę%fóď!a¸Ěš<52
EDIT4:
Finally I managed to make it work
I added request header and it returns proper JSON in body
I don't know why it has be added only in lazarus and not elsewhere.