SUMMARY
Once a plasma session becomes locked, it cannot be unlocked.

STEPS TO REPRODUCE
1. Log into a plasma session.
2. Allow the screen lock to engage or lock screen with Ctrl-Alt-L
3. Enter password to unlock session.

OBSERVED RESULT
After entry of password, screen widgets disappear showing just the wallpaper then after a second the screen widgets redisplay prompting for a password.

EXPECTED RESULT
Screen should be unlocked and desktop displayed.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.16.90
KDE Frameworks Version: 5.62.0
Qt Version: 5.14.0
Kernel Version: 5.3.5-arch1-1-ARCH
OS Type: 64-bit
Processors: 4 × Intel® Core™ i5 CPU M 430 @ 2.27GHz
Memory: 7.5 GiB of RAM

kscreenlocker 5.16.90-1

ADDITIONAL INFORMATION
System up to date with latest kde-unstable, testing and community-testing packages.

From journalctl:

Oct 11 16:41:29 richards-aspire audit[6220]: ANOM_ABEND auid=1000 uid=1000 gid=1000 ses=2 pid=6220 comm="kscreenlocker_g" exe="/usr/lib/kscreenlocker_greet" sig=11 res=1
Oct 11 16:41:29 richards-aspire kernel: kscreenlocker_g[6220]: segfault at 18 ip 00007f7a414cc351 sp 00007fff8226d6b0 error 4 in libQt5Qml.so.5.14.0[7f7a4143c000+305000]
Oct 11 16:41:29 richards-aspire kernel: Code: f2 0f 1f 84 00 00 00 00 00 89 d8 48 83 c2 02 c1 e0 05 29 d8 89 c3 0f b7 42 fe 01 c3 49 39 d4 77 e8 4d 01 fe 41 0f b7 06 31 d2 <48> 8b 75 18 66 89 44 24 0e 89 d8 f7 75 08 89 d0 4c 8b 24 c6 49 89
Oct 11 16:41:29 richards-aspire kernel: audit: type=1701 audit(1570808489.447:57): auid=1000 uid=1000 gid=1000 ses=2 pid=6220 comm="kscreenlocker_g" exe="/usr/lib/kscreenlocker_greet" sig=11 res=1