How can I "reset" a user's profile without rebuilding or restoring the whole machine?

If new users profiles are landing to C:\Users and not D:\Users, you can enforce the profile redirection to D drive and hence this should resolve the issue of user profiles.

-> HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList -> ProfilesDirectory , set its value to D:\Users

You can re-create the user profile once the previous one is corrupted.

-> net user username /logoff -> Delete the corrupted profile:

Delete D:\Users\username
Delete any temporary profile in C:\Users\username
Delete the user's SID entry in HKLM\SOFTWARE

-> Recreate base D:\Users folder with correct permissions:

md D:\Users icacls D:\Users /grant "Users":(OI)(CI)(RX,W) icacls D:\Users /grant "Administrators":(OI)(CI)F icacls D:\Users /grant "SYSTEM":(OI)(CI)F

-> shutdown /r /t 0

When dealing with corrupt user profiles in Amazon WorkSpaces, you have a few options to reset a user's profile without completely rebuilding the WorkSpace.

For your specific issue with Windows 11 BYOL WorkSpaces where users are getting "We can't sign you in to your account" errors and temporary profiles are being created in C:\Users instead of D:\Users, you'll need to properly recreate the profile on the D: drive.

Since you've already deleted the corrupt D:\Users<username> folder and the corresponding ProfileList registry entry, you now need to ensure new profiles are created on the D: drive rather than the C: drive. This is important because user data on drive C will be lost during any future rebuilds or migrations of the WorkSpace.

To redirect the user profile back to the D: drive, you'll need to modify registry settings to ensure new profiles are created in the correct location. This typically involves setting appropriate registry keys to redirect user shell folders to drive D.

If you're still experiencing issues after these steps, you might need to:

1. Create a new temporary user profile
2. Set up proper redirection to the D: drive through registry modifications
3. Ensure the user goes through the out-of-box experience again to properly create a new profile

Remember that if you leave user profiles on drive C, that data will be lost during any future rebuilds or migrations unless you manually back up and restore the data.
Sources
Rebuild a WorkSpace in WorkSpaces Personal - Amazon WorkSpaces
Upgrade Windows BYOL WorkSpaces in WorkSpaces Personal - Amazon WorkSpaces