I had the exact same problem today, after the updates. I actually had that new libPlasma.so.6.6.1 file in my /usr/lib so the update was probably OK however, just like the original poster mentioned, the system was looking for libPlasma.so.6 (which wasn’t there). I tried to symlink it to the 6.6.1 file and i got past the login screen but the desktop still did not come up. I only saw my mouse cursor. So i just reverted the updates.
EDIT: Resolved. You can update.