I was able to manually re-download all the extensions and the settings were automatically restored. However, it seems strange that Opera will not remember any web pages I've visited after a while. (I'm not using the private mode.)

Then it crashed again and I restarted Opera, everything was gone again. This time I was able to see the error message at launching, it says Opera is "unable to read user profile. Check to see if you have the privilege." I'm thinking maybe it has something to do with BitDefender for Mac.