The remote event receiver callout failed. Details: The HTTP request is unauthorized with client authentication scheme 'Anonymous'. The authentication header received from the server was 'Negotiate,NTLM'There was a remote event receiver binded to the AppInstalled event, the callout to this webservice failed.
This error seems indeed correct. The web application, hosted in IIS, was configured for Windows Authentication only.
So, this means that SharePoint calls this web service anonymously? If someone has a better explanation for this, please let me know.
I decided to open the website in IIS, and I changed the authentication for the folder that contained the webservices. Only for this folder I enabled Anonymous and disabled Windows Authentication. This works...
If someone has a better solution for this issue, please leave a comment.