I started re-synchronizing my folders to detect missing photos ... all went well for about 10 folders, but then after clicking on the 'synchronize' button, it crashed. The synch stops even before showing any picture in the screen. Just a black window.
I had to stop LR, and launch it again. I tried with different folders and always got the same issue.
I restarted the PC... everwthing, but still have the bug. I am unable to synchronize any folder now, while it was working very well 10 mn ago. See screenshots.
Have you logged in to the browser version at lightroom.adobe.com? If yes are you seeing exclamation points? You can deleted those to try resyncing them. You can also check and clear your Sync Errors folder under the catalog panel in Lightroom. Another option it to Rebuild Sync Data in your Preferences > Lightroom Sync (hold down option key) and click on Rebuild Sync Data button.