so I've had no issues at all with the latest LR Classic update on Windows 10, until last night when I had a second display attached to the notebook via HDMI. I only realized this was the issue after trying to fix the issue to no avail for an hour or two.
What happened was, I was editing images normally, but when I tried to sync the development settings among several images, either via ctrl+shift+s or the button in the library view, the program would just turn unresponsive at all. Not even its windows' minimize/maximize/close buttons would react. I had to end the process in task manager to close LrC. It was the same with a different catalog, after rebooting as well. Nothing in the event manager either.
At some point I figured out the only change to the previous days was the external display, and when I unplugged it, the syncing worked as it should, no freezing up anymore.
I only tested with syncing, no idea if copying or pasting the settings would've caused it as well
I didn't have the display attached for Lr, so it was no issue unplugging it, but maybe it's something worth looking to....