[400] Bad request - Parsing Error on /_field_caps

After _reindex, i have the HTTP 400 error with response below, when trying to call /_field_caps?fields=*
or /target/_field_caps?fields=*, whatever target is.
Anyone have an idea how to fix this ?
Did i miss something?
Elastic 7.9.3. thank you all.

{ "error": { "root_cause": [ { "type": "parsing_exception", "reason": "request does not support [query]", "line": 2, "col": 3 } ], "type": "parsing_exception", "reason": "request does not support [query]", "line": 2, "col": 3 }, "status": 400 }

Actually, i guess issue is not linked with _reindex. Looking a t the code, my feeling is that an index is not correctly build and the code falls in a parsing error, whatever index you ask, like if no scope is applied to process fields.
Any suggestion are welcome.