Hi @arevyhs, the action property on the form indicates that the default success page will be replaced with a custom page you create.
So for example entering the path e.g (/kontakty) of your custom page means that it must be relative to where the index.js file is located in the project root.
Therefore you need to create a kontakty.js file with the content you want to display.
If you don’t want to display a custom success page then remove the action attribute
Kindly check the resource below on how to add a custom success page with React
You can also checkout the Netlify Forum links below if you have not already done so since the problem is similar to yours. The suggestions there might be helpful.
If the above does not work, if possible kindly share a repository of your project so that I can help with the debugging.
