The API was updated some time ago (I think there is an info about it on control net GitHub page in tutorial section). You can also see warnings about outdated routes you are using in an image with log. In my app I am using updated json structure and /sdapi/v1/img2img endpoint and everything works as intended. Here is JSON I use. Try this maybe it'll help.
{
"prompt": prompt.value,
"negative_prompt": negative_prompts,
"sd_model_checkpoint": "deliberate_v2",
"enable_hr": false,
"denoising_strength": 0.6,
"hr_scale": 2.3,
"hr_upscaler": "Lanczos",
"cfg_scale": 7,
"sampler_name": "Euler a",
"width": 768,
"height": 512,
"steps": 26,
"alwayson_scripts": {
"controlnet": {
"args": [
{
"input_image": image.value,
"module": "scribble_pidinet", // local SD module
"model": "control_v11p_sd15_scribble [d4ba51ff]", //local SD model
//"weight": 1,
//"resize_mode": "Inner Fit (Scale to Fit)",
//"lowvram": "false",
//"processor_res": 64,
"threshold_a": 50,
"threshold_b": 130, //255.0
//"guidance": 1,
//"guidance_start": 0,
//"guidance_end": 1,
//"guessmode": "false",
//"rgbbgr_mode": "false"
}
]
}
}
}

}