Sorry that Lightroom Classic is not syncing hangs the app. Would you mind sharing the version of Lightroom Classic you're using?
Here are some general Lightroom Classic Syncing Troubleshooting Steps:
Go tohttps://lightroom.adobe.comand review the Sync Errors/Issues Album (upper right). Any unresolved syncs will show up as black thumbnails and have text instructions inside that tell you how to resolved the sync.
After all sync is resolved in Lightroom Web, launch Lightroom Classic and allow it to absorb the changes you've made. If your syncing count hasn't changed in 60 minutes proceed to next step.
Go to Lightroom Classic > Preferences>Lightroom Sync | Sync Activity and allow the dialog 30 seconds to refresh. Leave this dialog in place until all Uploading/Downloading/Pending Photo counts stop changing. Scroll down the list and find the stuck asset and remove it from your All Synced Photos collection. Allow time for sync to reestablish. Repeat if necessary. If the numbers do not move you can try the nuclear option.
In the location where your catalog is stored is a file called 'sync.lrdata'. Close Lightroom and rename sync.lrdata to sync.old. Restart Lightroom. Allow it to sync. If you have a large number of assets, this can take some time. I usually recommend doing this before you go to bed in case it is a several hour operation.
Someone from Adobe managed to walk through with me to resolve it. Found out that somehow, there were two screens which is on my computer. After dedicating to only one screen, the sync issue is resolved. Thanks for the quick response from Adobe on that same day and my apologies for not being able to annouce the resolution sooner.