After installing the latest Windows updates and rebooting, I got the following error:

*** STOP: 0x000000C4 (0X0000000000000091, 0x0000000000000000, 0xFFFFFF80002C4EFC0, 0x0000000000000000)

I could not boot into Safe Mode. I was able to boot into the Recover Console and uninstall the latest update that was still in the "Install Pending" state by doing this:

Determine what drive letter has your windows install. There's a good chance when in you are in the Recover Console command prompt that the Windows drive letter isn't your regular "C:\". Enter a drive letter to change to that drive, could be c:, d:, e: etc. Run "dir" to check what is in that drive. Note, the recover console environment, which might be "c:" also looks like a Windows install, but it isn't your regular install. It's a special Windows image for the Recover Console. Also try checking the "Users" directory contents to see if the users in it look correct.

d:

dir

cd Users

dir

The above should list contents that look like your regular Windows root. If so then use "d:" below, if not keep checking other drive letters. Once you've found the right drive letter replace "d:" in the commands below with that drive letter.

Next, make sure you have a "temp" directory or create one with:

d:

mkdir temp

Now uninstall the bad update. Note, this works for 64 bit Windows. Run this all on one line, replacing with the correct drive letter:

dism /image:d:\ /remove-package /packagename:Package_for_RollupFix~31bf3856ad364e35~amd64~~7601.24002.1.4 /norestart /scratchdir:d:\temp

Same line as above with different formatting if it's easier to read:

dism /image:d:\ /remove-package /packagename:Package_for_RollupFix~31bf3856ad364e35~amd64~~7601.24002.1.4 /norestart /scratchdir:d:\temp

Reported that the 32 bit package name is: Package_for_RollupFix~31bf3856ad364e35~x86~~7601.24002.1.4

Now reboot the computer and it should boot normally.

I do not know if that RollupFix is the same a KB4056894, but after removing that package the system booted successfully. The KB4056894 update is now listed as available in Windows Update again. I'm going to avoid installing that update, but it includes the fix for the latest Spectre and Meltdown security vulnerabilities. 

The STOP code appears to match this KB2816159, but when I tried to apply the hotfix, it says:

The update is not applicable to your computer.

Is KB4056894 causing this STOP: 0x000000C4 error?

Other reports have been able to use System Restore to restore to an earlier restore point successfully, but that might fail. Just because the recovery/repair disk says a restore doesn't work does not mean that it did not work. Some users receive an error message, 0x80070002, but when they tried to boot Windows again it worked and the KB5046894 was showing as a failed update. 

For me the CPU is a dual socket motherboard with two Dual Core AMD Opteron 285 chips. 

Based on other reports, this is effecting Windows 10, Windows 7, Windows Server 2008 R2, 32-bit and 64-bit installs for all older AMD CPUs. It is not related to the anti-virus registry key issue. Many reports are running standard Microsoft Security Essentials. AMD CPUs effected include Athlon, Sempron, Opteron and Turion:

• AMD Athlon X2 6000+
• AMD Athlon X2 5600+
• AMD Athlon X2 5400+
• AMD Athlon X2 5200+
• AMD Athlon X2 5050e
• AMD Athlon X2 4800+
• AMD Athlon X2 4600+
• AMD Athlon X2 4200+
• AMD Athlon X2 3800+
• AMD Athlon X2 BE-2400
• AMD Opteron 285
• AMD Opteron 2218
• AMD Opteron 2220
• AMD Semprom 3400+
• AMD Turion X

UPDATE: 2018-01-08

It's been three days now and still not knowledgement from Microsoft that this is an bug. This post has Views: 15014, Replies (56), I have the same question (445). This same error has been mentioned at the following sites:

TechNet: Blue Screen after KB4056894

Overclockers UK: Warning on KB4056894 (Intel/AMD) Fix caused Operton Bluescreen/STOP
AskWoody: Multiple reports of blue screens (BSODs) 0X000000C4 when installing the January Win7 Monthly Rollup KB 4056894

Reddit: Problems with Windows 7 Quality Rollup (KB4056894)

SevenForums: Windows 7 won't start after update on 5 Jan 2018

UPDATE: 2018-01-09

It's been four days and Microsoft has knowledged this is an bug and blocked the update for older AMD systems.

See Microsoft's official response here: 

Windows operating system security update block for some AMD based devices

Note: I unhid the update to see if it was blocked. After running "Check for updates", it is no longer showing up as a pending update for me.