Maybe there is an error in the created BASE64 string? I see two options to debug this:
Take the string, and copy it (on your PC) into Postman. =>If you test the connection with Postman, do you get the same result?
You probably know it, but just in case: Postman is a free cross-platform app.
If you see the problem even with Postman, a screenshot of the result could be helpful.
Or, in your C++ code, add one of these know valid OCR API base64 image strings. Does the ocr work then?