Hi Manuela,
This could be due to a plugin failure in CRM, unless you are calling the wrong Form Capture form action in the code. If you confirm that the form action you are posting to matches the "Form Capture" record in CRM then that means it's a plugin failure. If that's the case then you can try making a change to the Form Capture record in CRM where you just add a character to the name for example, save it, and then switch it back and save again. If the error persists then the next step is to recreate a new Form Capture record in CRM (which will create a new "form action") and then post again. Let us know what happens.