New
#51
manasa: just one of errors new in v1709, nothing more.
The mechanism is a little bit complicated: Lockscreen image reference is located in user's hive of registry (hkey_current_user), which is attached/loaded only if user is currently logged in. But Logonscreen image should be loaded even if no user is logged; that is why reference to Lockscreen is located under hkey_local_machine hive too. This works correctly for newly created accounts, but not for accounts existing before v1709 upgrade; they see mountain lake instead of their lockscreen image. The program logic is probably broken in this case.
So do not waste time by fiddling with pictures; deleting one registry key (unfortunately named by your SID but you can find its name easily) restores Logon screen functionality immediately; tested many times.