Display "Feed" uses a path but the path is undefined.

I just noticed that my feed views are broken. I did not change anything for a couple of months at that point, so it must have broken after updating the views module. I believe this is the reason as some weeks ago a views-update broke all my views (something ith SQLSTATE2422) and this has been fixed with the current version.

Anyway, opening my feed displays a blank page: http://goo.gl/8UU26
Please don't post name or URL of my site here. Use goo.gl or similar.

Opening the Feed settings in views gives this error. It was german, so I translated it merely.

A AJAX-HTTP-error occured
HTTP-Return-Code: 500
See below for debug information.
Path: /admin/structure/views/view/alben/preview/feed/ajax
Status text: error
answer text:

Since it was working before, this could be a bug. If not, please change.

Thanks in advance!