Almost every administrative action is prevented with "access denied" on a Windows Server 2008 R2

On a Server 2008 R2 (member of a domain but not a domain controller) we suddenly can't perform any administrative task. Every action (like net stop ..., whoami, ...) is denied with an access denied message. We can access the eventlogs but the only conspcious event is LSA 40961 (The Security System could not establish a secured connection ... )

Starting a program from explorer.exe works. Starting the same program from cmd.exe isn't possible (access denied).

What we tried so far

• Using local administrator instead of domain administrator => same behaviour
• Using elevated cmd.exe (right click run as administrator)
• Trying to disable uac (not possible due to access denied)
• Analyzing with sysinternals procmon => procmon can't be started
• anlyzing eventlogs => no conspicuous events

Edit
The attempt to disable UAC by the registry fails with the registry error: Error writing the value's new contents

Any ideas?