PUT with node-fetch: Unexpected token M in JSON at position 0

I want to update a filed according to relations in another field. GET the products works. Then I iterate over products and want to set the field entries.
When I test this in postman with a concrete ID instead of categoryIDs, it workes, but not in my nodejs-script. categoryIDs is an array of IDs.

console.log(JSON.stringify({data:{catalog_categories:categoryIDs}})) // => {"data":{"catalog_categories":[1]}}
            const dataProducts = await fetch(process.env.STRAPI_URL + '/api/product/' + product.id, {
                method: 'PUT',
                headers: {
                    Accept: '*/*',
                    'Content-Type': 'application/json',
                    Authorization: `Bearer ${process.env.STRAPI_TOKEN}`
                body: JSON.stringify({data:{catalog_categories:categoryIDs}})
            }).then( responseProducts => responseProducts.json() ).catch(err => console.log(err));

This gives a syntax error: Unexpected token M in JSON at position 0

If I change body: JSON.stringify({data:{catalog_categories:categoryIDs}}) to body: {"data":{"catalog_categories":categoryIDs}}, I get back:

  data: null,
  error: {
    status: 400,
    name: 'SyntaxError',
    message: 'Unexpected token o in JSON at position 1'

what is ok, because body is not stringified. But why is the stringified version not working?

Forgot the plural ā€œsā€. The API endpoint is /api/products not /api/product