Huge problems when updating to 4.4.1 – misc.php

Hello,

We are having huge issues when updating WordPress from 4.3.2 to 4.4.1.

Front end works but when trying to log in to admin we get this error message:

Fatal error: Call to undefined function wp_removable_query_args() in /wp-admin/includes/misc.php on line 900

To get into the admin we tried to change the misc file back to the old one. That worked fine.

We then tried in numerous ways to install the update again. Through removing all files/folders (which we were supposed to) and then placing the new once back. we tried with updating in dashboard. Nothing seems to help.

We are really stuck.

Does somebody know how to fix this issue?

If not. Who at wordpress can actually help with this?
We always have good support from plugin authors. But this is actually the first time we experience problems with the core. I don’t know who to turn to with this issue.

I am really thankful if you drop me a line!