Comment 4 for bug 1523261

The following messages on the javascript console look suspicious:

(https://*******.spdns.de/owncloud/core/vendor/jquery-ui/ui/jquery-ui.custom.js?v=96c6637dd73906ebc167790a0e192a44:319) Uncaught ReferenceError: jQuery is not defined
(https://*******.spdns.de/owncloud/core/js/jquery.avatar.js?v=96c6637dd73906ebc167790a0e192a44:108) Uncaught ReferenceError: jQuery is not defined
(https://*******.spdns.de/owncloud/core/js/backgroundjobs.js?v=96c6637dd73906ebc167790a0e192a44:23) Uncaught ReferenceError: $ is not defined
(https://*******.spdns.de/owncloud/apps/files_sharing/js/external.js?v=96c6637dd73906ebc167790a0e192a44:20) Uncaught ReferenceError: OCA is not defined
(https://*******.spdns.de/owncloud/apps/files_videoviewer/js/viewer.js?v=96c6637dd73906ebc167790a0e192a44:100) Uncaught ReferenceError: $ is not defined
(https://*******.spdns.de/owncloud/core/js/lostpassword.js?v=96c6637dd73906ebc167790a0e192a44:2) Uncaught ReferenceError: OC is not defined

Could it be that your owncloud setup is incomplete, thus preventing you from logging in? Can you log in to it from another browser?