Dropbox usually errors on start up when there's a problem with syncing a large number of files in your account. Here are a few potential reasons why that might be happening:
Poor internet connection - If you have a slow internet connection, it could be affecting the speed of syncing multiple files to Dropbox.
Too many files in the account - When you have too many files in your Dropbox account, it can cause files not to sync properly.
Corrupt file - If you have a corrupt file that's causing Dropbox to error, you can try deleting and re-uploading it or using the trashed file recovery feature to get rid of the corrupt file.
Antivirus or Firewall software - Sometimes, your security software could be blocking Dropbox from working properly. Try turning it off and then restarting Dropbox.
If you continue to have issues with Dropbox errors on startup, it's best to contact the Dropbox Support Team. They'll be able to help provide further troubleshooting steps or account specific help. You can reach out to them here: https://www.dropbox.com/support.