@Antonio Martinez Guerrero Apology for the delay and inconvenience due to this issue. Generally, the error is observed if your function app is stopped or there is an issue with your function app.
As per a response here from one of our product group engineers generally, the Function App can't be stopped for the ARM APIs to work properly.
If there is an issue with your function app then I will suggest you to review the application insights logs to get more details on the error/exception or you can also review the Diagnose and solve problems to resolve any issue related to your function.