http_referer error

Yesterday I wanted to create a new post.

The tinyMCE Advanced editor is not working. You can see the input only in blue when you highlight it. All menus disappeared.

In addition it is not possible to load any media.

The css of the dashboard is corrupted. And calling posts ends up in error messages:

[07-Oct-2017 16:14:49 UTC] PHP Notice:  Undefined index: HTTP_REFERER in /mnt/web120/a2/76/52551976/htdocs/wp-content/themes/oblique/page.php on line 33
[07-Oct-2017 16:16:50 UTC] PHP Notice:  Undefined index: HTTP_REFERER in /mnt/web120/a2/76/52551976/htdocs/wp-content/themes/oblique/single.php on line 19
[07-Oct-2017 16:17:10 UTC] PHP Warning:  Illegal string offset 'weite' in /mnt/web120/a2/76/52551976/htdocs/wp-content/plugins/code-snippets/php/snippet-ops.php(433) : eval()'d code on line 45
[07-Oct-2017 16:17:10 UTC] PHP Notice:  Uninitialized string offset: 0 in /mnt/web120/a2/76/52551976/htdocs/wp-content/plugins/code-snippets/php/snippet-ops.php(433) : eval()'d code on line 45
[07-Oct-2017 16:17:50 UTC] PHP Notice:  Undefined index: HTTP_REFERER in /mnt/web120/a2/76/52551976/htdocs/wp-content/themes/oblique/archive.php on line 55
[07-Oct-2017 16:21:30 UTC] PHP Notice:  Undefined index: showf in /mnt/web120/a2/76/52551976/htdocs/wp-content/plugins/ultimate-social-media-plus/libs/sfsi_widget.php on line 102

What might be the reason for this behaviour. I already switched off all plugins. It did not help. Neither emptying the cache nor using another theme helped.

I am totally confused and helpless.

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