I would suggest you to try following methods and check if it helps.
a. Press Windows + R keys together, type gpedit.msc in run window and press Enter.
b. In local group policy editor, under computer configuration, click to expand administrative templates.
c. Expand control panel and then click personalization.
d. Right click "Force a specific default lock screen image" and click edit.
e. Select enabled, click apply and then click ok.
f. Similarly, Right click "Force a specific Start background" and click edit.
g. Select enabled, click apply and then click ok.
h. Close local group policy editor and restart the computer.
I hope this information helps.
Please do let us know if you need any further assistance.
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