To investigate more on this issue, could you please provide more information on this.
1) What is your Lightroom Classic version ? 2) Was Lightroom syncing any photos when you turned on sync (like uploading/downloading) ? 3) How many assets are there in your catalog?
Also, can you try this workaround and let me know if Lightroom Classic crashes or not :
1) Create a text file and name it 'config.lua' 2) Add this command in the file, DigestManager.dontCalculateDigest = true 3) Save the file 4) Go to Lightroom > Preferences > Presets > Show Presets. 5) Go into the Lightroom folder. 6) Paste this config.lua file there 7) Quit and Relaunch Lightroom again.