I'm seeing the same thin even with libkscreenlocker5 5.19.5-2 from experimental. The locked screen is completely black, and wiggling the mouse shows the cursor but nothing else.
My workaround to unlocking the session is to switch to a TTY shell by ctrl-shift-2, and using 'loginctl unlock-session'.
smime.p7s
Description: S/MIME Cryptographic Signature
