[Workaround] Not able to login to their user profile - Build 20226

Some of the Windows Insiders had reported that in the recently released Windows 10 Insider Preview Build 20226 in Dev channel were not able to log into their User's profile. Here's a workaround released by Microsoft for the same. 

EDIT: This scenario could impact users in the Dev Channel (only) on builds 19593 through 20226.  The same workaround applies to any user in this build range in the Dev Channel who experiences this scenario.

The following message was seen by the affected users:

 

Workaround:  

The workaround applies only to builds 19593 through 20226 on the Dev channel.  It does not apply to Retail users, or users in the Beta or Release Preview Channels..

*If your account is an Administrator account on the affected device, follow these steps to bypass this issue:

1. When you see the “We can’t sign into your account” dialogue, click the Close button.
2. Open an elevated Command Prompt window.
3. Enter and submit the following commands one line at a time:
   • cd c:\users\%username%
   • takeown /F ntuser.dat*
   • icacls ntuser.dat* /reset
   • icacls ntuser.dat* /setowner "SYSTEM"
4. Sign out.
5. You will stop seeing the above dialog on your next sign-in.

Please note that the steps assumes your Windows installation is defaulted to the C: drive.  If you have installed Windows on a different drive letter, please replace c: with the proper drive letter for your installation.

*If your account is NOT an Administrator account on the affected device, follow these steps to bypass this issue:

1. When you see the “We can’t sign into your account” dialogue, click the Close button.
2. Open a standard Command Prompt window.
3. Enter the following command:
   • Echo %username%
   • (Make note of the returned username, you will use it in step 6)
4. Close the standard Command Prompt window.
5. Open an elevated Command Prompt window.
6. Enter and submit the following commands one line at a time:
   • cd c:\users\username
   • takeown /F ntuser.dat*
   • icacls ntuser.dat* /reset
   • icacls ntuser.dat* /setowner "SYSTEM"
7. Sign out
8. You will stop seeing the above dialog on your next sign-in

 If the above steps doesn't resolve the issue, then you will have to rollback to previous build to bypass. Then you will have to reinstall build 20226 and you will not encounter this issue the second time.

Microsoft Windows Insider Team has identified the cause of the bug and a fix is under development.

(source: Microsoft Answers Forum)