I am getting invalid_url again and again with /save_url, https://api.dropboxapi.com/2/files/save_url

Hm..🤔 I just checked it and everything seems valid. In first case I received:

{
  ".tag": "complete",
  "name": "#1135_GOLF-P-WE-AU-BLACK.eps",
  "path_lower": "/2023/november/#1135/golf-p-we-au-black/#1135_golf-p-we-au-black.eps",
  "path_display": "/2023/November/#1135/GOLF-P-WE-AU-BLACK/#1135_GOLF-P-WE-AU-BLACK.eps",
  "id": "id:ElL7W-JPrdMAAAAAAABbFw",
  "client_modified": "2023-11-28T09:48:28Z",
  "server_modified": "2023-11-28T09:48:29Z",
  "rev": "60b335227a2e322ab554a",
  "size": 82890,
  "is_downloadable": true,
  "content_hash": "e84f56b1b7802b8cc4be23d765222a27b7286852f672a077c918f82b9cbae0e7"
}

 Of course, It appears as expected:

The same when talking for the second case (after your typo correction):

{
  ".tag": "complete",
  "name": "#1134_GOLF-P-WE-AU-BLACK.eps",
  "path_lower": "/2023/november/#1134/golf-p-we-au-black/#1134_golf-p-we-au-black.eps",
  "path_display": "/2023/November/#1134/GOLF-P-WE-AU-BLACK/#1134_GOLF-P-WE-AU-BLACK.eps",
  "id": "id:ElL7W-JPrdMAAAAAAABbEg",
  "client_modified": "2023-11-28T09:42:34Z",
  "server_modified": "2023-11-28T09:42:35Z",
  "rev": "60b333d0821f622ab554a",
  "size": 251734,
  "is_downloadable": true,
  "content_hash": "80b133e0c090344bf04204c78b396fac3ef591122cd343ffd1722b7db8c82ab0"
}

To be able see "what is actually going through the wire" you need to inspect the actual queries (not just your logs). It's easy using netcat runing on your local machine and directing your queries there (replacing https://api.dropboxapi.com with http://XXX.XXX.XXX.XXX:8080). Here "XXX.XXX.XXX.XXX" is you public IP address and 8080 appropriate port to listen on. I suppose you know how to redirect such a query. In such a way you can see exactly what's going out of your code and what's wrong (not just supposings).

Good luck.