A HTTP 404 usually means, that something does not exist. Is it possible that not all products are in your Magento2?
If this is the case, then you need to filter (remove) the non-existing products first before passing them to the Magento2StockUpdate step.
Error during request 28:
You could check which SKU is in row 28 of your spreadsheet. And then check if there is something special with that sku.
This cookbook describes this process how to recognize new or existing products.
No, you need to filter (remove) the SKUs before, because otherwise you would flood your Magento with useless API calls. It can be done with the approach described in the cookbook.
Basically the approach is using 2 Flows:
1. Fetch all Products using Magent2GetProducts and store them in a Datastore using SpreadsheetDatastoreWriter
Now you know all products you have in Magento and can apply filtering. Run this flow e.g. once per day / at night, so it syncs new products from your Magento.
Flow 2: (your current import)
Use a a SpreadsheetMapper with the Lookup function (in german Querverweis) All Products which are not in your Magento should return a "No Record found".
Use a SpreadsheetFilter to do remove the "No Record found" rows.
Now you should only have rows which also exist in your Magento.
Proceed with your update.